Pressure Relief Valve is a valve term used to determine the pressure of a particular system that is controlled by the valve so that the valve is directly proportional to the pressure of the system, so opening the valve is not surprising, but gradually if the pressure gradually increases.
The Pressure Safety Valve is a valve term that prevents the system from being overloaded by a particular pressure. This valve is suddenly opened to maintain the system. If the pressure exceeds the specified value, the valve will almost completely open.