PIC PWM voltage regulator to build a variable V power

should I use pid logic inside PIC? Don't know how to use pid
in voltage regulation.

I found it is hard to get steady output V, in a such control system.
Output V is always changing, changing!!!

I tried to get constant current source also. Same thing, unsteady output!

how to control voltage regulator properly by PIC?

I would suggest taking the PIC outside the voltage regulation loop, and using the PIC just to generate a reference voltage. Smooth this reference voltage, then use discrete output voltage regulation that is based upon the reference voltage.