Manual: Manual mode is a mode on the camera that lets you manually select the iso, exposure, and aperture. No factor is automatically set.

TV: TV is similar to Av mode except this time the manually adjustable factor is the shutter speed. The aperture is automatically set along with everything else to the surrounding light as deemed by the camera.

Av: Av is aperture priority mode. Basically it automatically sets the camera to shutter speeds and all other factors the camera thinks fit the surrounding light. This enables you to adjust the aperture manually by hand.