does the duty point of a pump in performance curve change automatically depends the system head?
for example, i have pump with best efficiency at Q = 10 m3/min and H = 5 m , but i use the pump in a system which
has H system = 7 m, and from the performance curve we have Q = 6 m3/min.
So theoritically we have actual Q 6 m3/min right? because the H system is 7 m
How about the butterfly valve? is it true in performance test, butterfly valve could be use as
the system (change) by control the valve in order to make performance curve?

A centrifugal pump always operates along its performance curve. If you increase the head (discharge pressure) it will pump less. If you lower the discharge pressure, it will pump more.

You typically try to select a pump that operates at its best efficiency point but unless you have a completely fixed system and can build a custom pump then you can only get as close to the best efficiency as you can with a given selection.

The way performance curves are developed is to change the discharge pressure. You can use any type of restriction including a butterfly valve to create this restriction.