This varies depending on which acne scar treatment you are having done. Overall, acne scarring treatment is not painful. However you may feel some discomfort when undergoing laser skin resurfacing. This is because this procedure uses light and heat to treat damaged skin cells. This should resolve itself within  a few days.
A potential side effect of laser skin resurfacing is infection, which can cause pain. If you experience pain during or after any of your treatments, you should inform your dermatologist.