What is the hook up difference between I/P and EP positioner for control valve?

In the case of I/P Positioner, the electric signal is translated to a pneumatic signal first (usually from 0.2 to 1 Bar) by the mean of an I/P converter. That pneumatic signal then will act on some sort of internal pneumatic elements inside the positioner (diaphragms, pneumatic relays, nozzles, etc.) to give an output to the valve.

While in EP all that conversion is done internally inside the positioner itself.

We'll need in the I/P positioner case:

• 2 Air Supply inlets (1 for the I/P and the other for the Positioner)
• Pipe connecting I/P outlet to the Positioner inlet.
• Pipe connecting Positioner outlet to Valve Actuator inlet.

While in the case of EP Positioner, we'll need:

• 1 Air Supply inlet to the positioner.
• Pipe connecting Positioner outle to the Valve Actuator inlet.

I/p is your Pneumatic hook up and EP is your electrical hook up

A detailed answer is already added by Ahmed.

I/p convertors converts a current input signal for eg. 4-20 ma to pneumatic siganl which controls the opening and closing of a control valve and e/p have simmiliar operations as those of i/p but it can be either a voltage input based or current input based. 

I/P is current to pressuer while EP is voltage to pressure.