A positive displacement pump makes a fluid 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 of the discharge since the cavity decreases.
The most typical type among centrifugal pump is the radial flow pump. … The primary difference between these kinds of pumps and centrifugal is that positive displacement pumps will move liquid at the same rate regardless of the strain on the inlet end and centrifugal pumps won’t