From the Tux in a Hat dept.:
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. Virtualization using the KVM hypervisor is supported on wide variety of guest operating systems...

redhat.com