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 part expands and the liquid flows out of the discharge since the cavity decreases.
The most typical type among centrifugal pump is the radial flow pump. … The main difference between these types of pumps and centrifugal is definitely that positive displacement pumps will move liquid at the same speed regardless of the pressure on the inlet end and centrifugal 
pumps will not