Today I needed to convert an OVA to a raw disk for Xen. OVAs are original video animations that are usually released after a popular anime series, as well as being a bundle for Open Virtual Appliances.
OVAs are just tars, so you can extract them as such:
# tar xvf target.ova
From there, you can convert the resulting images to raw. I wrote how to do for vmdks exactly one year ago. Scary timing.