A positive displacement pump makes a fluid 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 part expands 
and the liquid flows from the discharge because the cavity decreases.
The most common type among centrifugal pump is the radial flow pump. … The main difference between these types of pumps and centrifugal is usually that positive displacement pumps will move liquid at the same quickness regardless of the pressure on the inlet end and centrifugal pumps won’t