What is the pain is involved when having a total hip replacement operation.?

Mild to moderate. The pain with hip replacment surgery is a function of the damage to the tissues as one tries to accurately place the new joint. Surgeons using techniques to minimize the damage will have less pain post op. The direct anterior approach to hip replacement offers the least damage to the muscles when performed by a surgeon experienced in this technique.
Variable. A hip replacement surgery is normally less painful than a knee replacement surgery. Depending upon your pre-operative pain level, often, there is a different, but more tolerable pain after total hip arthroplasty. Always, however, everyone perceives pain differently.
Not too bad... I recommend using an epidural for the surgery and then transition to oral medications a few hours later. I'd also suggest talking to your doctor about giving your a few different types of pain medication immediately before surgery, which helps reduce the amount of pain you experience afterward. Fortunately, hip replacements do not hurt that much - most of our patients walk the same day day.