artificial heart valve simply means substituting natural heart valves with artificial ones. There are four heart valves. So there are 4 types of artificial heart valve replacement. Mechanical heart valve just means there is an artificial value, but doesn't specify where. Mitral valve is one of the 4 heart valve so Mechanical mitral valve is indicating the site where the artificial heart valve is placed.
So usually INR goal is 2-3, but when you have a mechanical valve you need an INR goal of 2.5-3.5.