There is no DV to USB cable, as DV is in this case just another term for Firewire, and USB + Firewire are not compatible, thus no "converter" cable exists. Most likely the camera also has a mini-USB port, thus it is able to stream bad quality video via USB.
In fewer words: there is nor Firewire (DV) to USB cable. To get DV quality, you need a Mac or Windows/Linux PC with a Firewire port.
mini-USB
Firewire 4-pin (DV, i.Link, IEEE 1394)