Bootable VM Images?

Virtual Machines are great for testing things on and running software your native operating system won’t support. Sometimes, though, you actually want to be able to boot into them rather than run them on top of another operating system. If you run Linux for a main OS and Windows for gaming, a VM isn’t going to work that well for you due to all the extra overhead of running two operating systems at once, not to mention poor graphics frame rates on many VMs. Shouldn’t it be possible to create an image that you could both boot into like a regular partition, but also run inside a virtual machine? So far I’ve found nothing that can do this…