What is difference between motion and relative motion
Every object which is in motion is actually in relative motion. Suppose, you found an object which is in motion but not in relative motion. Ask yourself how do you know that the object is in motion? You will realize that the object is in motion with respect to you, which means the object is in relative motion.
I am not sure if you got the concept correctly or you did not frame your doubt properly. You seem to be confused with relative motion with motion.
Motion, as you said, is change in position with respect to time but the definition of relative motion you gave is not really correct. Relative motion is the change in position with respect to a point in space with time. In fact, relative motion and motion are exactly one and the same.
Any time you talk about motion , its relative motion. You say something is in motion because it is moving with respect to you which means, say what, it is in relative motion with respect to you.
There is no such thing as absolute motion.