If I stick to one of the standard, well-supported VM disk images (like a raw image, or VDI, VMDK, ...), are Linux VMs typically easy to move between VM environments? E.g., between (say) VirtualBox and KVM, or VMWare and Xen? I'm talking here of fully virtualized environments, not paravirtualization requiring support within the guest OS.

It seems to me that the kernels in most Linux distributions these days are configured to...keep an open mind and detect things at boot time, so you don't have the issue that you sometimes have moving a Windows VM from one virtualization system to another (I'm thinking particularly of HAL issues that Windows has, like ACPI vs. non-ACPI; I've also just had Windows VMs generally acting strangely when moved from VMWare to VirtualBox, for instance).

I'm looking for a general answer, but if it helps, specifically I'm mostly going to be doing this with Ubuntu 8.04 LTS and 10.04 LTS guests. But that could change.