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 part expands and the liquid flows out of the discharge since 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 definitely that positive displacement pumps will move fluid at the 
same swiftness regardless of the pressure on the inlet end and centrifugal pumps will not