I don't think that this is possible, as well the Xen installer says this when you try to install the server software in a VM. http://imgur.com/q1eoAYx
What you could do though is run Xen as your host and then RHEL as your guest.
Update: Here is an image showing it working we have Xenserver 7 as our host (XS7-01) and we have two guests Windows 7 Pro x64 (CL-03) and RHEL 7 x64 (rhel7-01).