what is the mechanism of hydro-mechanical system in power control unit that uses follow up feedback?
There is a position control loop in there that opens the servo valve until the required position is reached, then closes it. The loop uses position of the actuator as a feedback signal, measured by a position transducer, usually an LVDT. Open loop servo valves just generate an actuator velocity, not very desirable when flying a plane.

A more robust system used on aircraft has the hydraulic actuator moving together with the flight control surface. The pilot control stick is connected to the servo valve, and when he deflects the stick he opens the valve and oil starts to flow. This makes the flight control surface deflect. The actuator moves with it, in doing so closing the valve again.

You mention half powered controls in a comment - I assume you mean boosted flight controls, where the pilot directly deflects the control surface but hydraulics help in overcoming the aerodynamic forces. These work in the same way as described above, with a follow up mechanism that moves the actuator until the valve closes, but the mechanical linkage is different. 

Here is a schematic diagram of a hydraulic actuation system with a mechanical servo valve:



You can see that the cylinder is anchored to the airplane structure. The diagram is a bit primitive, but the gist of it is:


The elevator is at rest, not moving.
The pilot gives a control input, which opens the servo valve (the little box on top of the cylinder).
The piston deflects because the servo valve is open.
The pilot input linkage is attached to the piston as well. Because of the movement of the piston, the servo valve closes again.


This mechanical feedback system is robust because there are very few electronics that can fail. In this set-up, elevator deflection velocity is proportional to stick deflection. Other transfer functions are possible bit programmed feedback loops and transducers.

This system is fully irreversible: airforces on the elevator push against the cylinder piston and are not fed back to the mechanical linkage at all.