A positive displacement pump makes a liquid move by trapping a set amount and forcing (displacing) that trapped volume in to the discharge pipe. … Liquid flows in to the pumpas the 
cavity on the suction side expands and the liquid flows out from the discharge because the cavity decreases.
The most common type among centrifugal pump may be the radial flow pump. … The primary difference between these types of pumps and centrifugal can be that positive displacement pumps will move liquid at the same speed regardless of the pressure on the inlet end and centrifugal pumps won’t