From http://www.redhat.com
RHEL 5 Virtualized guest types:
Fully-virtualized
Para-virtualized (highest performance)
Fully-virtualized:
Requires features provided by Intel Virtualization Technology and AMD-V processors
Available for RHEL 3, 4, 5, and other third party operating systems.
Para-virtualized:
Any 32-bit x86 (with PAE support), 64-bit x86, or Itanium2 system processor
Available for RHEL 4 (update 5) and RHEL 5.