A WH (normaly) doesn't have a PRV...that, if installed, is in-line somewhere just after the home's main shutoff. If the valve has a lever on the top, it is probably the T&P safety valve. That can release water under two conditions (unless defective): if the internal pressure exceeds its limit, or the internal temperature exceeds its limit. Unless the thermostat has failed, and the water is exceptionally hot, it is more likely pressure. If your home uses a closed system (it has a check valve in it so house water can't push back to the street), it should have an expansion tank to moderate the water pressure when it expands due to heating it. First thing I'd do is get a screw-on water pressure gauge with a tattle-tale hand (peak holding indicator). They cost about $10. Leave it installed for 24-hours and see what the peak pressure is. If that's normal (stays below 80psi), and the temperature of the water at the WH isn't excessively hot, the valve probably needs to be replaced.