I've just looked in a couple of Pilot's Operating Handbooks:
Piper Seminole: They call it a "Pilot's Storm Window" (I'm not sure what problem it is supposed to cure in a storm)
Mooney: They call it a "Pilot Window"
I always thought DV stood for "Direct Vision" but the logic of this thread now makes me think that "Direct Ventilation" is more likely.
If someone has successfully landed an aircraft while the DV window provided the only outside view, then I'm very impressed.