The on/off valve is fully opening or closing valves in the pipeline, it can isolate a fluid totally and provide tight shut-off.The control valve is to control and regulate the flow according to the process condition, its shut off capabilities is not as good as the on/off valve.
Control valves regulate and control the flow, pressure, the temperature of a flowing fluid, such as gas, steam, water, or the chemical mixture by opening or closing internal passages. They control the amount of flow in processes. The valve stem moves, altering the size of passage, and this increases, decrease or holds steady flow.
Valve positioners are the devices that are used to put a valve in the right positioner by increasing and decreasing the air load pressure on the actuator. The valve positioner senses the exact position of the valve and communicates to the actuator to drive the valve.