I have created PVMs in LVM partitions in the past. I assume that
HVMs would be similar. I believe I was using CentOS 4 at the time, and I
had to use the complete /dev path instead of the shortcut you get used to
with most distros (including CentOS). Also, I was using standard XenSource
rpm-installed manually-configured xen in runlevel 3, so I don't know if you
can do that through the Fedora tools or even with the Fedora Xen. However,
you might give it a shot. Unfortunately, I am not currently messing with
LVM, so I can't give you any kind of detail on this (such as an example
complete /dev path vs shortcut).
Dustin