From Xen

A Brief History of Virtualization

A Brief History

First of all, this is not the Bible of virtualization, i.e we don't like to speak a lot about everything. This is a look at virtualization with the Xen Project hypervisor. In the computing world, when we speak about “virtualization,” its mean that you want to create a virtual version of something, this can be a that can be a program, an OS, etc.

The Very Beginning of Virtualization: The Mainframe

Virtualization is not a new technology. In the 1960s, it was used by mainframe computers and made by IBM. Jim Rymarczyk was a programmer that joined IBM in the 1960s as a mainframe expert and he invented virtualization. At that time, IBM used CP-67 software. It was a control program of CP/CMS that was a virtual machine operating system developed for the IBM System/360-67. CP/CMS was a time-sharing operating system that became popular due to its excellent performance. CP-67 was the first of IBM’s attempts to virtualize mainframe operating systems.

CP-67 gave customers the ability to run many applications, and was essentially the first “spark” of virtualization. CP-67 was replaced by CP-40, which was an operating system for the System/360 mainframe. The CP-67 was the second version of the IBM hypervisor. The early hypervisor used a conversational monitor system (CSM) that was a simple interactive operating system. The IBM hypervisor became a commercial product in 1972 with VM technology for the mainframe and nowadays it is used as z/VM. The z/VM is a full virtualization solution for the mainframe market.

The main advantages of using virtualization on mainframes is that we are able to share the overall resources of the mainframe between all users. If you want more information around this, I recommend to look at Unix OS history. Unix OS is an example of virtualization at the end user level, and will give you a good understanding of the first steps towards application virtualization.

Application Virtualization at Sun Microsystems

Application virtualization began in 1990 when Sun Microsystems started a project called “Stealth.” It was a project for preparing a better way to write and run applications. The name of the project changed many times and finally in 1995 Sun Microsystems rename it to Java. Java called computers run different operating systems with a rich set of applications. Java let the developer write applications that run on all OSes via Java Runtime Environment (JRE). You just need to install JRE and your program will run. The JRE is composed of many components like Java Virtual Machine. When you run a Java application, then your application runs inside of the Java Virtual Machine and you can consider it as a very small OS. For more information, you can see: “”.

Why and what is Virtualization?

We spoke about the history of virtualization, but lets define it more on the technical side: why is virtualization important and why should you use it?

Using Moore’s Law: “The number of transistors in a dense integrated circuit doubles approximately every two years.” This has become a rule to estimate the future of integrated circuits, but some people would estimate it to actually be every 18 months. Please see below picture from Wikipedia:

Figure 1: Microprocessor Transistor Count

We must accept that hardware becomes cheaper and cheaper and advanced and more advanced. Compare your old computer with your current computer. What do you see?

For example, my old computer had 256MB Ram with Pentium 4 1.7 GHz, but my current computer is Intel Core i7 with 8GB Ram. As you see, hardware becomes faster and faster and cheaper too, but are you always reaching hardware capacity? I bet most of your CPU and Ram is unused, and you are just using energy. So what should we do? It's just a PC, but how about servers?

Servers use more energy and need extra equipment for maintenance. Nowadays, machines just use 10 or 15 percent of their capacity. Remember Moore’s Law, after two years we have a more powerful hardware, but why? We can't use all hardware capacities and most of our equipment is wasted. Virtualization solves this concern as we can use all of our hardware capacity with virtualization.

In the computing world, x86 virtualization means hardware virtualization for x86 architecture. This technology allows multi-OSes to use a x86 processor resources in a safe and efficient way. Early, x86 virtualization was a complex software technology because it filled the lack of hardware virtualization. However, in 2006, Intel and AMD introduced limited hardware virtualization with the names Intel (VT-x) and AMD (AMD-V). Both of them allow virtualization.

The AMD-V was the first AMD generation virtualization that developed under the code name "Pacifica" and the company introduced it as AMD Secure Virtual Machine (SVM) but changed it to AMD-V.

In 2006, AMD released the Athlon 64, the Athlon 64 X2 and the Athlon 64 FX and all of them use this technology and were the first AMD CPU generation that support virtualization. In 2005, The Intel company released two models of the Pentium 4 (Model 662 and 672) as the first Intel processors that support VT-x. In 2015, All Intel CPUs support VT-x and most motherboards.

In short, virtualization allows you to use all the capacity of your server to service users.

Types of Virtualization

If you do some research, then you can find different types of virtualization. Some resource tells you that there are three main types of virtualization, others four types of virtualization. The three types often referenced are client, server, and storage. The four types that are often mentioned are operating system virtualization, server virtualization, storage virtualization and hardware virtualization.

More details on each below.

Operating system Virtualization or containers?

Client virtualization refers to virtualization on a desktop or laptop computer. Operating System (OS) virtualization means the movement of the main desktop OS in a virtual environment. In this method, The OS is hosted on a server, i.e. one version on the server and copy of that is present on each user. The user can modify his/her own OS without impacting other users.

Containers can help you move an application from one computing environment to another and the Kernel of OS will be run on hardware with several isolated guest virtual machines. Popular containers are Docker, VagrantUp and LXC. Containers can help you with overhead and performance. Containers are convenient, but there can be security challenges.

Server Virtualization:

Server virtualization means moving a physical server into a virtual environment. This kind of virtualization will solve data centers concerns. Nowadays, servers can run more than one server simultaneously helping to reduce the number of servers. IT companies like this because they can gain more control over growing their server farms. Server virtualization is critical for IT companies because with server virtualization they can add more machines --- if they can't add more machines then they can't respond to customers needs. Server virtualization is very popular in the web hosting and databases areas. These areas have many benefits because each server can run its own OS and rebooting each server can't impact on other servers.

Storage Virtualization:

Storage virtualization means combining multiple physical HDDs into a single virtualized storage. Another name for it is “cloud computing.” Cloud computing can enable better functionality and features. Storage virtualization can help administrators easily with backup, archiving and recovery.

This technology can be private, public or mixed. Private is hosted by your company, public is out of your company like “DropBox”, “Microsoft One Drive”, “Amazon S3” and a mixed environment is a combination of both.

In cloud storage, data is stored in logical pools and physical storage and physical environments owned by the provider. The biggest responsibility of the provider is to keep the data available and accessible and the physical environment must be protected and running always. Companies can save energy and money if they chose this route, Data availability and protection is better and this storage can be used for copying VM images or importing them. Additionally, storage virtualization has a better backup because data is copied in different location around the world.

Hardware Virtualization:

Hardware virtualization means taking the components of a real machine and making them virtual. Another name for it is platform virtualization, which refers to creating a VM that behaves like a real computer with an OS. Softwares that runs on these kind of VMs are separated from hardware resources because virtualization hides the physical characteristics of users. For example, you can run Microsoft Windows 10 on a Linux machine or vice versa. The Microsoft Windows 10 that is running on a VM can't understand that it is virtualized and thinks that it is a real machine. The software that creates a VM on hardware is called a hypervisor or Virtual Machine Manager. The software is separated from hardware resources.

Different Types of Hardware Virtualization exist :

  • Full Virtualization: The VM simulates hardware in a way that the Guest OS does not require any modification to run.
  • Partial Virtualization: The VM simulates multiple instances of hardware -- meaning the entire OS cannot run in the VM. This kind of hardware virtualization is important because for space constraints.
  • ParaVirtualization: The VM doesn't need any hardware simulation, but offers a special API that can modify the Guest OS. As you see, OS modification is needed thus OS source code must be available. This technology was introduced by the Xen Project team. It is so useful because you don't need any virtualization extensions on the host CPU. It enables virtualization on hardware that does not support hardware-assisted virtualization.

Virtualization and Security

In the virtualization world, you can make a VM, convert and isolate the VM from the Host . For example, when you launch a virtual network between VMs and use Virtual HDDs for testing and forensics. First of all, virtualization adds additional layers of complexity and therefore monitoring and finding security vulnerabilities becomes more difficult. A hacker must do more research in order to discover more vulnerabilities. Virtualization can provide isolation and it is the core feature of network virtualization. A network that is virtualized is isolated from other virtual networks and also physical networks. The important thing is that no Firewall, ACLs and… are required for this isolation. Virtual networks are isolated from the physical infrastructure because traffic between hypervisors is encapsulated and the physical network operates in a different address space. A good example of it is that networks can be IPv6 and virtual networks can be IPv4 or vice versa. This protects underlying physical network from attacks.

In networking there is a concept by the name of “Network Segmentation”. Network Segmentation can split a computer network into subnetworks and make each of them a network segment. Network segmentation can improve security and performance. It provides security because when an attacker gains access to your network, segmentation provides a good control for limit access to the network. This can be implemented by a hypervisor switch or OpenvSwitch.

To note, these features can make some mistakes. For example, securing a virtual machine is same as a physical machine can configure your VM in a bad way, For example, it can with open unnecessary ports putting your VM can be at risk.

Fortunately, the Xen Project hypervisor provides a good security feature that will be discussed later in the book. A good example of an OS that created for security via Xen is “Qubes-OS”. For more information about this project see “”. Another good example is “Sandbox.” The Sandbox is a mechanism for separating running programs. You use can sandbox for executing untrusted code or programs from untrusted users and websites. Sandboxes are a good example of virtualization that running suspicious program without any harm to the host device.


A few examples of sandbox implementations is “SELinux,” “Apparmor,” “Virtual machine,” “JVM”, “Sandboxie,” and some features in the browser like “Chromium”. Sandboxie is an isolating program that was developed by the Invincea Windows OS. It allows users to run and install applications without modifying your drive:

Figure 2: Sandboxie

You can download it from “”.

For Sandboxing under Linux see “Mbox” at “”.

As you understand, virtualization have some advantage and disadvantage and with the passage of time. hackers and malware authors have found ways to bypass it. A good example of this is “Paranoid Fish”. You can see more information about this project at “”.

Containers vs Virtualization

The next topic is about containers. If you remember we told you something about “Operating-system-level virtualization,” but we want show you that this technology is various from virtualization.

Containers are not a new technology and Unix used it many years ago, but in recent years this technology has become more popular with the advent of Docker, Vagrantup and LXC

In 2004, Docker teamed up with companies like Canonical, Google, Red Hat and Parallels to create a standard that allows containers to work within Linux namespaces and control groups without any admin access and offer a better interface for all Linux Distro. This allows many containers to run in a single VM. 

Before this, you had to use a VM for each application and separate them from each other, but now, you don't need it; you can run all of them in one VM environment. Thus, you don't need many VMs on a machine. A big problem with VMs on a machine was overhead and containers solved it.

Containers also solved a problem that system administrators and developers faced with it for many years: sys admins and developers would produce a tool, but couldn't run it on some environments because of version mismatching of any library or packages installation problems. Docker solved this problem to by making an image of an entire application, with all its dependencies and allowing the user to move this to their intended target environment.

What do you think? I guess you are thinking that you can solve this problem via VMs too. Well, taking an image of an entire virtual host and launching it on the target takes time. Containers are so light weight making it so your Apps is ready in few seconds.

Containers with all the advantages have disadvantages too, and one of the biggest problems with them is “Security” particularly with cloud environments. The containers share the same hooks into the kernel. This is a problem because if any vulnerabilities exist in the Kernel then an attacker can get into your containers. Containers can't provide a secure boundary like VMs. If you do some search about Docker vulnerabilities, then you can find some interesting topics. For example, a vulnerabilities in Docker let attackers escape the system and gain full access to the server. A tool like “Clair” ( can help you about analysis of vulnerabilities in apps and docker containers. For more information about Docker security you can look at “” and “”.

The five security concerns when using Docker are :

  1. Kernel exploits
  2. Denial-of-service attacks
  3. Container breakouts
  4. Poisoned images
  5. Compromising secrets.

The main idea behind a hypervisor was to emulate the underlying physical hardware and create Virtual Hardware for you. You can install your OS on top of these virtualized hardware. In below Diagrams you can find the different between containers and VM:

Figure 3: Docker Architecture
Figure 4: Virtual Machine Architecture
Figure 5: Different between Virtualization and Containers

As you understand, If you need security, then your option is VM otherwise select containers.

Open Source Linux Virtualization Software

Now, the time has come and we want to look at some virtualization software and familiarize you with it.


The Xen Project was born at University of Cambridge as a research project by Ian Pratt and Simon Crosby. Ian Pratt co-founded XenSource and the first version of Xen made in 2003. The Xen Project was supported by XenSource Inc and in October 2007, Citrix bought XenSource, Inc. Citrix bought Xen, but continued to support the free version of Xen. Additionally the sold the commercialized version of Xen too for enterprises, it is called “Citrix XenServer." In 2013, the free/open source version of Xen moved under the auspices of the Linux Foundation and “Xen” changed to “Xen Project” and differentiated from older name. With this changed, project members like Amazon, AMD, Cisco, Citrix, Intel, Oracle and more support the Project.

Xen Project is a hypervisor that manages CPU, Memory and other hardware for all Virtual Machines and the most privileged domain. In Xen Project refers VMs as “domains” and privileged domain as “dom0”. The dom0 is the only Virtual Machine that has direct access to hardware. From dom0, The hypervisor can manage domU and unprivileged domain) can be launched.

The dom0 is a version of Linux or BSD and domains can be other OSes like Microsoft Windows.

The Linux Kernel from version 3.0, includes supports of Xen for dom0 and domU in the Kernel. Xen Project can support live migration for VMs and also support sload balancing that prevents downtime.

Load balancing distributes workloads across multiple computing resources, such as computers, clusters, network links, CPUs and Disks. Load balancing increases reliability and availability through redundancy.

Xen supports five types of Virtualization : HVM, HVM with PV drivers, PVHVM, PVH and Paravirtualization.


KVM or Kernel-based Virtual Machine is a virtualization for Linux Kernel that turns the kernel into a hypervisor. KVM needed a CPU that supported hardware virtualization. If you remember, we spoke about it (Intel VT-x or AMD-V). In KVM, The Linux Kernel acts as a hosted hypervisor (Type 2 Hypervisor) that is a Virtual Machine manager that is installed as a software on an existing OS. KVM, simple manages and improves performance in virtualized environments. KVM creates a VM and coordinates CPU, Memory, HDD and other hardware equipment via the host OS for our VM.

KVM can support a wide range of OS like Linux, Windows, Solaris and even OS X. A modified version of QEMU can use KVM to run OS X. KVM, doesn't do any emulation, iIt uses /dev/kvm interface that a is a userspace for:

  • Setup address space for guest VM.
  • Creating a Virtual Machine.
  • Reading and writing VCPU registers.
  • Inject and interrupt into a VCPU.
  • Running a VCPU.

For BIOS, KVM uses SeaBIOS. It is an open source implementation of a 16-bit x86 BIOS that support standard BIOS features.

You may ask yourself, what are KVM benefits?

Security: Because, KVM is built on top of Linux kernel, it can use capabilities of Selinux. With this benefit, KVM can provide Mandatory Access Control security between virtual machines.

QoS: As we said, KVM is part of the Linux Kernel thus a VM does not have differ with another program that running on Linux thus administrator can define thresholds for CPU, Memory and better guarantee QoS for VMs.

Open Source: KVM is an open source solution that provides open source benefits and makes interoperable solutions available. As you guess, new hardware features and support for the new generation CPU architectures can fix in it. For example, 64-bit ARMv8 architecture targets the server and mobile platform and KVM support it, thus, KVM-on-ARMv8 is a key virtualization technology for many markets.

Other benefits are full virtualization and near-native performance.

With these advantages, KVM has some disadvantage too. For example, Complex Networking, Limited Processors and CPU Virtualization Support.

You can find a good Performance benchmarks about Xen and KVM at “”.

KVM was acquired by Red Hat in 2008.

Below is a figure from Wikipedia about KVM architecture :

Figure 6: KVM Architecture


OpenVZ (Open Virtuozzo) is a technology for operating system-level virtualization in Linux that allow you to run multiple isolated OSes on a server. It is a container like LXC. OpenVZ can't prepare full virtualization like Xen Project and it's just a path for the Linux Kernel and can run only Linux. Its disadvantage is that OpenVZ shares the same architecture and Kernel version. OpenVZ Virtual Machines are jailed containers and are not true VMs, like the Xen Project. OpenVZ improved and added a good feature: in the old version of OpenVZ, each virtual environment uses a same file system that isolated by “chroot” but in the current version of OpenVZ each container has its own file system.

A good advantage in OpenVZ is that memory allocation is soft. This means that a memory that's used by a virtual environment can't use by others. I guess you heard “ virtual private servers (VPSs)”. OpenVZ called VPS too, and in the computing world, The VPS is a VM that is sold as a service. A VPS running its own OS and customer has full access to it and can install anything on it. VPSs in some ways are equal to dedicated servers, but their prices are so lower than dedicated servers. VPSs performance compared with dedicated servers are much lower because they use shared hardware. I found a diagram about it and you can see it below :

Figure 7 : VPS

In April 2006, there were two good features for OpenVZ released and they are “live migration” and “Checkpointing”. With live migration in OpenVZ you can move a container from one physical server to another without shutting down your container. The checkpointing means that if a container freezes, you can save it as a file on disk. Then you can move this file to another machine and restore it.

LXC 2.0 released and if you Google it, then you can find some articles about converting OpenVZ to LXC. For example, “”.

You can't run Windows OS on OpenVZ, but I found a trick about it :


The first version of Linux-VServer was released in Oct 2001. The Linux-Vserver is like OpenVZ, but adds operating system-level virtualization capabilities to the Linux kernel. It is used for abstract computer resources like File Systems, CPUs, Network and Memory based on security contexts and process that can't launch DoS attack on others.

With the Linux-Vserver, you can create many independent VPSs that run on a physical server at full speed and shares hardware. As we said, VPSs are independent and all services like SSH, Databases, Mail, etc. can start without or with a very small modification. VPSs are isolated and each VPS has different authentication. Linux-Vserver like OpenVZ uses the “chroot” utility for providing security and can run Linux guests. The Linux-Vserver does not emulate any hardware and the goal of it is isolating applications -- this isolation done with Kernel. Linux-Vserver can integrate with Grsecuirty to provide for providing better security. It has some advantage and disadvantage. Disadvantages include:

Advantages Include:

  • Not have any overhead.
  • Because of common file system, the back-up is easier.
  • Networking is based on isolation not virtualization, so there is no additional we have not any additional overhead for packets.
  • You can run it in a Xen guest.

Disadvantages Include:

  • The host Kernel must be patched before installing. More information here.
  • Clustering and live migration are not supported.
  • Because of Networking is based on isolation, each VPS can't create its own internal routing or firewall.
  • There is an additional patch for supporting IPv6.
  • When you shut down a guest then the IP is brought down on the host too.

It has other features like “resource sharing”, “resource Limiting”, “good disk scheduling”, “hide packet counters” and…

For more information about building guest systems you can see “ “.


The first version of VirtualBox was released by a German company with the name Innotek GmbH as a closed source software, but it was available for free.

In January 2007, Innotek GmbH released an open source version of VirtualBox (VirtualBox Open Source Edition (OSE)) under GPL version 2. Sun Microsystems acquired the company in February 2008; Sun Microsystems was then acquired by Oracle in January 2010. When Sun Microsystems bought Innotek company, the company changed VirtualBox to Sun xVM VirtualBox. The xVM was a product line from Sun Microsystems that addressed virtualization technology on x86 platforms : Sun xVM hypervisor that was a component of Solaris OS and provided the standard features of a Xen-based hypervisor for x86 and Sun xVM Server that was based on xVM hypervisor project. The overall goal here was to support Microsoft Windows, Linux and Solaris as guest OSs. After Oracle acquired Sun, the changed the name again to Oracle VM VirtualBox.

VirtualBox or VB can install on many platforms, including Linux, Windows, Solaris, FreeBSD and OS X. VB can support many Guest OSs on Linux and Windows platforms. For better performance and graphic resolution, VB uses "guest additions" packages that make VB more powerful. It is a CD-ROM image under .iso format with the name “VboxGuestAdditions.iso”. After installing this package the Guest OS has better performance and features like:

  • Mouse pointer integration
  • Shared folders
  • Better resolution and video support
  • Seamless windows
  • Shared clipboard

If you want to enable features like “Support virtual USB 2.0/3.0 controller,” “PXE Bootfor Intel card, ” “disk image encryption” and “RDP” then you must use a close source pack for VirtualBox with the name “VirtualBox Extension Pack.” It is a file with “.vbox-extpack” extension and easy to install. Just double click on it.

VB provides “full virtualization, and has good features, however some of them are e although some are experimental features. A few examples of experimental features include:

  • 64 bit Guest (hardware virtualization support like Intel(VT-x) and AMD(AMD-V) are required)
  • Snapshots.
  • Seamless mode
  • Command line interaction
  • ICH9 chipset emulation
  • EFI firmware
  • Host CD/DVD drive pass-through

A diagram of VirtualBox Architecture is below :

Figure 8: VirtualBox Architecture

Compare Virtualization Software

In this section, I want to compare virtualization softwares. We will not compared all virtualization software, but will look at some of the most important.

Name Full Virt ParaVirt OS Virt (Containers) Host OS Architectures License

GNU/Linux, Unix-like, FreeBSD i686, x86-64, IA64, PPC GPL V2

GNU/Linux, Unix-like i686, x86-64, IA64, PPC, S390 GPL V2

Linux i686, x86-64, IA64, PPC, SPARC GPL

Linux Everywhere Linux is GPL V2

GNU/Linux, Windows, OS X x86, Solaris, FreeBSD i686, x86-64 GPL V2
Citrix XenServer

No host OS x86, x86-64, ARM, IA-64, PPC GPL V2
VMware ESX

No host OS i686, x86-64 Close Source
Oracle VM Server for x86

No host OS IA-32, x86-64 GNU GPL

No host OS x86, x86-64 ????

In the computing world, a VM is an emulation of a computer; it stands for a virtual machine. Virtual Machines based on computer architecture work as a real or virtual computer. As we said, different kinds of Virtual Machines exists and each of them provides different features and abilities. The physical computer is called the “Host” and Virtual Machine called the “Guest” and guest OS is what runs on your real computer.

You can manage your hypervisor and containers via different GUI and web managers: