No, they're usually the multiple piston / "swash plate" type...as are most electrically driven pumps. The output is controlled by varying the angle of the plate and thus stroke of the pistons. The plate will move relative to the demand on the system. This way a pump can be running but if the demand is minimal, actually moving little fluid ( compared to a high demand situation ). The pump's purpose is really just to supply fluid _volume_...the pressure coming from the restrictions that the various user systems place on the system as a whole.
the pumps are running to constant pressure at their outlet. if more demand, the pressure tends to decrease, this makes the plate adjust providing for higher stroke of the pistons, thus retoring nominal pressure but at higher flow.