The ability that defines Nighthawk Scavenger's power applies in all zones, not just the battlefield.

Nighthawk Scavenger counts card types, not cards. If the only card in your opponents' graveyards is a single artifact creature card, Nighthawk Scavenger will be 3/3. If the only cards in those graveyards are ten artifact cards and ten creature cards, Nighthawk Scavenger will still be 3/3.

Card types that can appear on cards in a graveyard include artifact, creature, enchantment, instant, land, planeswalker, and sorcery. Legendary, basic, and snow are supertypes, not card types; Vampire and Rogue are subtypes, not card types.