Xen doesn't officially offer a P2V solution for migrating existing physical Windows instances into XEN VMs. Their answer is a link to two vendors that offer XEN support for their P2V products. Both companies, however, charge a few hundred bucks per-migration, which makes it a bit expensive for the one-off migrations here and there (in both vendors' defense, the P2V functions aren't the main purpose of their products). I've used the following steps to perform migrations of Dell-based Windows 2003 Server instances into XEN virtual machines simply using NTBackup, without having to purchase any "P2V" tools/licenses.
These steps basically mimic how one can recover a Windows server onto new hardware after a failure, with some extra precautions taken to avoid interrupting the existing physical instance until it's ready to be replaced.
To learn more and to read the entire article, please refer to the rest of the article at its source: geekpoet: Manual "P2V" migration of a Windows server into a XEN VM instance