Future value is a result of the compounding of interest earned on the present value, or starting amount. As time progresses, the periodic interest is added to the starting amount and the next periodic interest amount is calculated based on that.
As such, the amount of interest earned over the entire time period can be found by subtracting the present value from the future value:
Amount of Interest = FV - PV
Click this link to learn more about compound interest and other Time Value of Money concepts.