Xen provides a UUID for running domains, but this UUID is only available from
dom0, not domU. Management applications (such as RHN) need a way to associate
the domain attached to a given dom0 with the running instance.
For fully virtualized hosts, we will not be able to expose this data in /proc,
so it will need to be exposed via DMI.
See bug #189497 for UUID exposure in paravirtualized domains.
I've tested that xen > 3.0.2-31 RPMs provide SMBIOS data to fullyvirtualized
guests. Please run 'dmidecode' in a HVM guest & confirm that the UUID data
satisfies your requirements.
This is long since fixed in both Fedora & RHEL.