I am interested in setting virtual machines/servers which I'd like to be able to port between different virtualization systems. An example of what I mean would be to install Debian under VirtualBox, then after shutting it down, move the image over and run the same system under KVM.

The thing this would require is hard drive image format cross compatibility (e.g. non-live sharing). Of course, I could just use plain raw non-compacted images, but that eats space (and time when transferring them around). But can different virtualization engines support a common compacted hard drive image format?

Both KVM and VirtualBox will read and boot the same images. The biggest problem that you are going to run into is device drivers, since KVM (mostly) provides multiple access to actual hardware, whereas VirtualBox spoofs false devices and provides a cross-over API into the host system.

Because of this, you'll be limited to fairly generic, flexible configurations on virtual Linux hosts, and virtual Windows hosts will be given fits.

I won't be moving hibernating systems. Life on a new VM platform will begin with a boot up, so as long as the device drivers can be detected and selected as needed, it would think this would be OK. I just have to be sure I have the ones for both platforms like I would if moving a hard drive from one machine to another with unlike devices.

It shouldn't be anything really complicated, and it will just be Linux and BSD ... mostly Linux.