convert vm from xen 3.2 opensource to citrix xenserver 5.5

Did this a few years ago.  There really is no clear upgrade path.  Best thing we found to do was create a new VM on the XenServer, but not load an OS into it.  Take the newly created storage, and mount it to another convenient VM that can share it out via NFS.  Mount the new storage via NFS to the old machine.  Make sure you install the latest Xenserver kernel into the old VM, and set it to boot via grub.  Do a wholesale copy from the old VM to the new.  Shut down the old VM when complete.  Umount the new storage from NFS and from the temporary VM, then give it back to the new VM that was created and boot up.  Install XenServer tools and you're done.

Note you can also use the VM in HVM mode and not need to run the xenserver kernel before the conversion.  

Ehr sorry but I've posted a reponse but now that I review the question there is none. SOrry for potential doubleposting but here it is in general:

The guest I'm trying to move is a Fedora Core 5-installation from way back and it's not supported in the newer XenServer as a client (I think, at least not that old?), so I'd like a way to run it in HVM-mode so that I can slowly migrate all the webclients on that specific machine to a newer, supported VM. How would I go about for getting it to run in HVM mode? It has a very old Xen-enabled Fedora-Core kernel so it is "partially" aware it's virtualized but I don't know how this kernel would work in the newest version of XenServer.

Any thoughts, ideas, go-abouts?

Excellent I'll try that out! THANKS!

Very helpful! Many thanks!