Well, I wrote a while back about portability of PV virtual machines, but I've also just had some experience with portability of fully-virtualized VMs (HVM in Xen language). I have a mixture of ESX and Xen for my virtual machines here, and I like to be able to move VMs back and forth. I've recently moved a couple of Linux-based VMs from ESX to Xen, which turned out to be very, very easy. ESX, it seems, creates two files for each disk - the descriptor file, and a "flat" file. The descriptor file simply tells ESX about the disk and then points over to the flat file for the data. The flat file is nothing more than a raw file - it has a partition table and then the data in each of the partitions. So, to move a VM from ESX to Xen, all you have to do is copy out the Flat file to the Xen box and point Xen at the flat file. You can also tell Xen to use the previous MAC address from VMware.

Going the other way isn't all that hard, either. You can copy the raw file from a Xen VM over to the ESX box, then you just need to create a descriptor file for it. I haven't done this, yet, though I'm sure I will, soon. I doubt you can do it from the VI Client, which will mean a little command line work, but I always liked getting my hands dirty on the CLI. The only trick to this will be that VMware limits the MAC addresses you can set on VMs, which means you may end up with a new MAC address if the original VM has a Xen-ranged MAC address. This is slightly annoying, though understandable, and shouldn't cause too many problems.

Anyway, that's it for now - just nice to know that I can move these things around without too much trouble! Using an NFS datastore would help, too, because I could access VMs from both ESX and Xen. I'm just concerned about NFS performance (though VMFS performance isn't anything to brag about), and I'm concerned about getting an NFS server setup that's not a single point-of-failure.