In the United States ‘Passivhaus’ is often spelled ‘Passive House’. This leads to some confusion, as most people associate ‘passive’ with passive solar houses. While a Passivhaus and a passive solar house have a lot in common, they are not the same thing.
First, passive solar refers to a design technique, not a certification. Any house that takes advantage of sunlight to warm the house is in some way a passive solar house. That said, most people think of homes with a lot of windows and steep roof overhangs when they think of passive solar. These homes are designed to absorb as much heat as possible in the winter, but have a roof that deflects summer sunshine that would overheat the house.
Passivhaus design takes advantage of passive solar strategies, but goes further. It also requires very stringent air infiltration (tightness) scores, and dictates exactly how much energy can be committed to heating and cooling. Basically, Passivhaus designers take passive solar design and add it to a a stable of energy saving strategies.