Xen and Virtualbox etc both support a range of different virtual machine image formats.

These are: vmdk, vdi, qcow & qcow2, hdd & vhd.

Without any bias toward a particular product, I'm wanting to know what are the advantages and disadvantages of the various formats both from a features perspective, robustness and speed?

One piece of info I discovered in a forum post was this:

"The major difference is that VDI uses relatively large blocks (1MB) when growing an image, and thus has less overhead for block pointers etc. but isn't ultimately space efficient in the sense that if a single byte is non-zero in such a 1MB block the entire space is used. VMDK in contrast uses 64K blocks, and thus has more management overhead and generally a bit less disk space consumption What offsets this is that VDI is more efficient when it comes to snapshots."

You might be thinking, I want to know this because I want to know which format to choose? Not exactly, I'm developing some software which utilises these formats and want to support one or more of them. Simplicity, large disks and ease of development are my main drivers.