Are there tools for migrating from one virtual machine type to another? E.g let's say I have some Xen virtual machine and like to make it run under KVM. I know that qeumu has tools which can be used to "migrate" such machines, but how about:
Xen -> Kvm
Kvm -> Xen
Xen -> VMware (server?)
The format you're looking for is OVF (Open Virtualization Format). It looks like its support in KVM is controversial, but otherwise I think it's been received pretty positively. (Disclaimer: I work for VMware, one of the co-creators of OVF)