Red Hat RHEL 5.7 has increased virtualisation speed for KVM and XEN

• http://docs.redhat.com/docs/RHEL5/5.7_Release_Notes/virtualization.html

According to The Register Red Hat are replacing Xen with KVM in RHEL 6 !

One of the main goals for RHEL 6 was to make managing virtual servers as easy as managing physical machines, which means the bulk of the new software features are found in KVM. It also means that Xen is gone, though that’s hardly surprising since Red Hat purchased Qumranet – creators of KVM – back in 2008.

Red Hat RHEL VIrtualisation Webinars

Red Hat Enterprise Linux 5.4

now includes full support for the Kernel-based Virtual Machine (KVM) hypervisor on x86_64 based architectures. KVM is integrated into the Linux kernel, providing a virtualization platform that takes advantage of the stability, features, and hardware support inherent in Red Hat Enterprise Linux.

Note
Xen is the default hypervisor that is shipped with Red Hat Enterprise Linux. As such all configuration defaults are tailored for use with the Xen hypervisor. For details on configuring a system for KVM, please refer to the Virtualization Guide.

Important
Xen based virtualization is fully supported. However, Xen-based virtualization requires a different version of the kernel to function. The KVM hypervisor can only be used with the regular (non-Xen) kernel.

Warning
While Xen and KVM may be installed on the same system, the default networking configuration for these are different. Users are strongly recommended to only install one hypervisor on a system.

Red Hat defends replacing Xen with KVM