I am sure that this is discussed on the xen lists from time to time. Here is one time that I am familiar with:
http://lists.xensource.com/archives/html/xen-users/2008-01/msg00521.html
RHEL is still slightly different case than pure open source xen since Red Hat applies patches that don't make it immediately into Xen. They also add some value add type things and support it, which are different than Citrix's XenServer (Express, Standard, Enterprise) -- formerly Xen Source's XenServer Expres -- in that they are typically open source additions. Citrix may very well base their XenServer product on RHEL or centOS or something similar, but I don't know that for a fact.
They also work differently, XenServer uses a dedicated machine and is access with a client console on another machine. While, normal installs of Xen, such as Xen on RHEL, are used on the machine on which they are installed, with remote management optional.
I don't have direct experience with RHEL, only centOS which is what RHEL is based on.