From DMV website:
If your registration has a full six months or more remaining before it expires, you may qualify for a partial refund. Simply complete an Application for Vehicle Registration Refund form (form FMS-210) and return it to DMV. Refunds generally take 30 days to process."
Form can be found here:
http://www.dmv.state.va.us/webdoc/pdf/fms210.pdf