Hi RequestedDeletion
I'm afraid we have absolutely no experience with xen. We use either native machines or Vmware. In either case we just run up the standard sark distro, which is a modified SME8b4 or a regular EL5. They use the standard Digium Asterisk & Dahdi RPMs from the Digium repo.
we've also played around with virtualbox running under Ubuntu and we have a version of SAIL running on that but again, its just a standard ISO install.
Sorry we can't help more.
Best
S