Can we use a control valve without a positioner
It would be used when the process is not being controlled for accuracy but just to get close to a number whether it be flow pressure or other forms. If you are talking about a globe style or v ball valve with an on/off actuator then without a positioner would either be to open a line to cause flow or close to stop the flow.
Actually, a control valve will operate as a modulated valve without a positioner attached to it but it will operate accordinfg to the I/P and of the bench set of actuator. It will not be as accurate without the positioner but will still function.
Valves used in HVAC usually do NOT have a positioner because of cost and the lack of need for precise action.