Some (all?) DV camcorders transfer the raw DV data over Firewire but recompress the video when tranferring over USB. DV camcorders are really meant to be used with Firewire.
I have some 11-year-old recordings on Mini-DV, recorded with a Canon Optura Xi A camcorder. I can't figure out how to transfer them to a PC. I'd be happy with just playing the videos on my PC screen ...