What is the difference between Pressure regulating valve (PRV) and pressure relief valve (PRV) ?

pressure regulating valves and and reducing valve both are user to control flow of liquids ,but the regulating vale we mostly use for more accurate pressure measuring at gas lines for good accuracy state .

And pressure reducing valve we used mostly for liquids for maintaining the set pressure  

A pressure relief valve opens when the pressure in a system reaches a certain level to avoid the pressure going any higher. It only operates occasionally.

A reducing valve is a device for restricting the flow of a fluid so that the pressure is reduced in a controlled way. It operates continuously. An orifice plate is the simple shape of pressure reducing valve