Can I record from my HD PVR to a DVD?
We will need a great many more details on your exact equipment before we can advise you.
Probably not, but like he said above it depends on what DVR. Tivo's will let you network anything not write protected to your PC and burn it any way you want, most others will not.
Depending on the make/model of your PVR and whether it uses a proprietary video format or standard one (MPEG etc.) you might be able to mod your equipment to allow you to connect the hard drive to your PC as a USB external drive. I was able to do this with great results for my Humax PVR-9200T (the built-in connectivity is slow and highly error-prone). Google is your best resource for this sort of info, and the bits shouldn't cost much.
Downgraded material will come out of analogue connectors maybe, but nothing more.
usb is mostly only for firmware maintenances.