From Xen

XenLinux is the Linux kernel with patches applied so that it will run on the virtual architecture presented by the Xen hypervisor rather than on real hardware. Since version 3.0 of the Linux kernel, the Xen patches have been applied to the upstream kernel. This means it can serve as the kernel of a domain, but it will no longer run natively on a machine. Although other operating systems have been ported to run on Xen, Linux is the only one that has been given the extra functionality needed to run in dom0.

See also : Hurd.