prv is a pressure relief valve. If the pressure gets too high for the particular application the prv discharges some water. Central heating systems are lablelled 3 bar but can go off from 2.5 bar upwards. If they fail to go off at 3 bar then the final safety device is that the cap blows off and we all get wet. That is a design feature. Other values of these valve exist for other applications.
plv is not an abbreviation I use. Maybe you are talking about a gosvenor or regulator.
pressure reducing valves often found on unvented dhw cylinders regulate the flow pressure to say 3bar for example. Pressure limiting valves are a less accurate valve and limit the pressure normally to 5bar. Pressure relief valves (prv's) open at a predetermined pressure (typically 3bar on ch) to prevent overpressurisation of pipework.
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