Citrix recently released a new open source tool to simplify virtual machines migration from the open source Xen hypervisor to its open source implementation XenServer and the upcoming Xen Cloud Platform (XCP).
The virtual to virtual (V2V) migration utility is written in Python and can either perform an offline conversion of a Xen VM into that XVA format that XenServer and XCP understand, or live streaming the VM to the two target platforms.
The tool supports both para-virtualized and hardware-virtualization-assisted (HVM) virtual machines but it still is in its early stage and Citrix warns about issues and bugs.