Hello,
If you're interested in running Xen 4.0 hypervisor/dom0 on RHEL6, take a look at here: http://wiki.xen.org/xenwiki/RHEL6Xen4Tutorial
It explains steps needed to rebuild Xen 4.0.1 src.rpm from Fedora on RHEL6, and how to fetch dom0 capable 2.6.32.x kernel from upstream git repository.
It also shows how to get libvirt/virt-manager working with Xen on RHEL6.
Hopefully it helps :)
-- Pasi