"The Linux kernel 2.6.20 and above included KVM. RHEL 5.5 (and
upcoming RHEL 6) supports KVM out of box and it has also been
ported to FreeBSD as a loadable kernel module. However, this
tutorial is tested on both CentOS and RHEL 5.5 only running 64 bit
Intel Xeon CPU (with Intel VT) and 64 bit kernels with SELinux
running in enforcing mode.
"XEN allows several guest operating systems to execute on the
same computer hardware and it is also included with RHEL 5.5. But,
why use KVM over XEN? KVM is part of the official Linux kernel and
fully supported by both Novell and Redhat. Xen boots from GRUB and
loads a modified host operating system such as RHEL into the dom0
(host domain). KVM do not have concept of dom0 and domU. It uses
/dev/kvm interface to setup the guest operating systems and
provides required drivers."