There's a new qemu disk format that's still in very early developement called QED, which stands for QEMU enhanced disk format.  The RFC for this new disk disk format was first published September 2010 last year on the qemu mailing list.  Although you could argue that there are enough qemu disk formats, qed tries to avoid some of the design pitfalls of qcow2 and can be considered a next generation qemu disk format. Qed is expected to be more performant than qcow2 and early tests are showing that it is meeting it's design goals of high performance. QED is able to achieve its high performance through a simple on disk layout.

QED is included with qemu version 0.14 so if you're running the most recent version of qemu-kvm you can start testing it using the qemu-img utility. The list of current features in QED include the following:

• open specification
• fully asynchronous I/O path
• strong data integrity
• backing files
• sparse files

Future Features still in development include the following:

• image streaming
• online defragmentation
• Trim
• parallel submission
• meta-data scan avoidance

You can read more about the qed disk format and features at the following qemu wiki page on qed.