If a virtual machine (VM) is being backed up as a whole, the VM -- whether it be Microsoft, VMware or Xen -- is completely encapsulated in just a couple of files. It is generally encapsulated in the virtual machine disk file. Essentially, they are configuration files which in each system is a little different.
Then there is also their BIOS configuration, their NVRAM and everything else that would be normally associated with a physical machine. It is a lot like taking a ghost image of a running physical server, except that a ghost image takes the operating system and files from a logical level. The VM image contains not only that hard drive image, but also the configuration for the hardware that was presented to the VM.
So, a ghost image only contains the operating system and it might get pushed to a piece of hardware that may have a different number of processes, a different amount of RAM or a different network card configuration. But, when you restore a VM image, it's not only the image of the hard drives, it's also its hardware configuration, its NVRAM, its BIOS settings and everything that is logically needed to bring that server back online.
This was first published in June 2008