Difference between Pressure Relief valve and Pressure Reducing valve 

Pressure Relief valve
Pressure relief valve will normally be in closed position and it will only come in line once pressure requirement increases and reaches the setting point of relief valve.
Pressure relief valve will have one pilot line to sense the inlet pressure.
Pressure relief valve will be internally drained as the outlet of relief valve will be connected with reservoir.
Pressure reducing valve
Pressure reducing valve will receive higher pressure at inlet but it will deliver the pressure, as per outlet pressure setting, on its outlet.
Pressure reducing valve will normally be in open position.
Pressure reducing valve will have one check valve to allow free flow in reverse direction.
Pressure reducing valve will be externally drained as the outlet of such valve will not be connected with reservoir so such valve could not be internally drained.
Pressure reducing valve will have a separate drain line directly from valve to reservoir.
Pressure reducing valve will have one pilot line to sense the outlet pressure.