Howtoforge posted a guide about convert an image-based guest to an LVM-based guest in Xen

This short article explains how you can move/convert a Xen guest that uses disk images to LVM volumes. Virtual machines that use disk images are very slow and heavy on disk IO, therefore it is often better to use LVM. Also, LVM-based guests are easier to back up (using LVM snapshots).

Xen: How to Convert An Image-Based Guest To An LVM-Based Guest