virtualbox.org

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I am Running Windows XP as host and Windows Server 2003 guest machines. I tried both VHD and VMDK seem to be close in performance but I'll have to do more testing.

Has anybody experience which format is better for Virtual Box in terms of speed and compatibility?

The native format of VirtualBox is VDI, and that is therefore the format with greatest VBox "compatibility". I don't know if the others offer very different performance (though I have doubts about "very"), but I do know that those formats are beyond the control of the VBox developers, so any ideas we might have about improving performance cannot be implemented in them.