A positive displacement pump makes a liquid move by trapping a fixed amount and forcing (displacing) that trapped volume into the discharge pipe. … Liquid flows into the pumpas the cavity on the suction aspect expands and the liquid flows from 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 certainly that positive displacement pumps will move liquid 
at the same acceleration regardless of the pressure on the inlet end and centrifugal pumps won’t