Just to clarify something, coz you sound very confused.
In Av mode, you choose the aperture (eg. f/8, f/11, etc) and the camera calculates what the shutter speed should be in order to get the exposure required.
In Tv mode, you choose the shutter speed (eg. 1/60s, etc) and the camera calculates what the aperture value should be in order to get the exposure required.
BUT BUT BUT if you set it Tv and 1/250s for example, and the place is quite dim, the camera goes up to the maximum aperture it can reach (smallest f-number), and cannot go anymore than that. So on the 18-55 kit lens, it might go to f/3.5 and the picture might still be too dark.