Is jaw alignment surgery for underbite very painful?

It can be. It depends on many factors but the short answer is, yes, it can be rough sailing for a few days. There is a fair amount of swelling and you will probably be on a liquid/blenderized diet for a few days at least.
Mild-moderate. Usually very nicely controlled with pain medication. Discuss this with your oral & maxillfacial surgeon.
After. The surgery is done under general anesthesia. Today's techniques and post-surgical medications make your recovery less complex than it was just a few years back. You will be very sore for a few days, but the soreness rapidly dimities. The end result will be a miraculous change for the better.
No. You are asleep for the surgery. When you wake up you will have pain (as you would for any surgery) but it can be managed with medication. You will need less medicine after the third day and you should not need any at all after a week.
It Depends on many. Orthognathic surgery, or corrective jaw surgery, is very routine for correction of an underbite. It may involve moving the upper jaw forward or moving the lower jaw back, or both. The amount of pain depends on many factors (multifactorial) including your age, the actual operation, your experience with pain and your tolerance of pain. Many young healthy patients have little pain which is manageable.
Jaw surgery pain. You will recieve pains meds that will help a lot. Be prepared to stay out of action for 10-12 days.

Related Questions

How painful is a jaw alignment surgery?

Jaw surgery. Believe it or not, not that painful. Most of the patients complain of stuffiness in the nose. But not pain.
Varies. Orthognathic surgery may involve surgery in one jaw or both jaws. Any surgical procedure is usually accompanied with some level of pain. Pain perception and how individuals react and respond to pain varies from person to person.