If I read your question correctly you have a Linux hypervisor (a Xen dom0) with Linux LVM volumes which are used as virtual disks for your Xen guests (domU). And you want to access the data in that LVM directly from the hypervisor, bypassing the Xen guest.
I don't use Xen anymore but I imagine the process is similar to what I do with KVM guests:
Shut the guest domU down. (accessing a filesystem from two different locations is asking for data corruption)
On the hypervisor run
kpartx which will scan the LVM for a partition table and create device entries for each partition found (where I assume have a Volume Group Guests and a LVM guestname-diskname which is the virtual hard disk for your domU)
[root@dom0 ]# kpartx -a /dev/Guests/guestname-diskname
If you had three partitions in your Xen guest this should result in 3 new device entries:
Then you should be able to mount a partition with a file-system simply with:
[root@dom0 ]# mount -t ext3 /dev/Guests/guestname-diskname2 /mnt
It becomes a little bit more complex if you used LVM inside the domU as well; then the partitions are of the type
8e Linux LVM and formatted as physical volumes which you can't simply mount yet and LVM trickery is needed. Instead of the mount command the steps become:
Hopefully you can detect the volume group that existed within the domU:
[root@dom0 ]# vgscan
Found volume group "Guests" using metadata type lvm2
Found volume group "VG-guestname" using metadata type lvm2 <== NEW!
Next you need to enable the volume group "VG-guestname" :
[root@dom0 ]# vgchange -a y guestname
lvscan should show the logical volumes that were created in your domU and the device mapper entries are typically created now as well, allowing you to do something like:
[root@dom0 ]# mount -t ext3 /dev/VG-guestname/guest-lvname /mnt
IIRC it wasn't possible in Xen but can happen with KVM, the inconvenient case where the name of a volume group used in the hypervisor is the same as one assigned in the guest.