Pressure Independent Control Valves (PICV) ensure a constant flow, regardless of pressure fluctuations upstream. Each PICV has an internal chamber which is kept at a steady pressure differential relative to the valve outlet. This is done by directing the inlet flow through a set of concentric sleeves before reaching the internal chamber. The sleeve openings narrow down with increasing pressure and vice-versa. PICVs are ideal for ensuring that whole radiator systems stay balanced. 

Differential Pressure Control Valves (DPCV) maintain a constant pressure differential between inlet and outlet. The upper and lower chambers of the valve are separated by a diaphragm, which is kept in balance by a spring-loaded piston. The piston contracts with increasing differential pressure, allowing greater throughput and thus bringing differential pressure back into equilibrium. A decrease in differential pressure produces the opposite effect. 

DPCVs are used to control pressure conditions in sub-branches and to enhance the valve authority of thermostatic controllers. They neutralise the effects of pressure instability, arriving either by surge or collapse.