PV on HVM

What is it?

PV on HVM is a mixture of paravirtualization and full hardware virtualization. The primary goal of PV on HVM is to boost performance of fully virtualized HVM guests through use of specially optimized paravirtual device drivers (also called PVHVM or PV-on-HVM drivers).

The table shows how PV, HVM and PV on HVM compare in terms of virtualization method:

	HVM	PV on HVM	PV
Boot Sequence	Emulated	Emulated	Paravirtualized
Memory	Hardware	Hardware	Paravirtualized
Interrupts, Timers & Spinlocks	Emulated	Paravirtualized (Emulated on Windows)	Paravirtualized
Disk & Network	Emulated	Paravirtualized	Paravirtualized
Privileged Operations	Hardware	Hardware	Paravirtualized

Performance

PV-on-HVM drivers are optimized PV drivers for HVM environments that bypass the emulation for disk and network IO. They also make use CPU functionality such as Intel EPT or AMD NPT support. Thus, giving you PV-like or better performance on HVM systems. You can find information about performance trade-offs in this presentation. In a nutshell:

• For workloads that favor PV MMUs, PV on HVM is similar to PV
• For workloads that favor nested paging (in hardware with Intel EPT or AMD NPT support), PV on HVM performs better than PV

Compatibility

The following links provide information about Xen Project PV on HVM compatibility with various operating systems:

• HVM on PV support in the Linux Kernel
• HVM on PV in (legacy) Linux distributions

Also See

• Information about using PV-on-HVM drivers
• HowTo on PV-on-HVM drivers
• Xen Windows HVM guests can use the opensource GPLPV drivers