From Red Hat Software:
A base Red Hat Enterprise Linux server provides fully-virtualized guest support for Red Hat Enterprise Linux 3, 4, and 5. Third party operating systems may also be hosted. Fully-virtualized guest support requires hardware features provided by Intel Virtualization Technology and AMD-V processors. Para-virtualized guests can be configured on any 32-bit x86 (with PAE support), 64-bit x86, or Itanium2 system. With Red Hat Enterprise Linux Advanced Platform the number of guest operating systems that may be hosted is unlimited, constrained only by physical hardware resources.