Denim vs canvas vs linen: Aren't they all cotton?

Denim is made of cotton in a twill weave. Canvas is made of either cotton or linen, sometimes (historically) hemp and made in a flat, tight weave. Linen is made of flax.

The courseness of the thread and the weave.

[*] Modern canvas is usually made of cotton or linen; although, historically it was made from hemp. It differs from other heavy cotton fabrics, such as denim, in being plain weave rather than twill weave. Canvas comes in two basic types: plain and duck. The threads in duck canvas are more tightly woven. The term duck comes from the Dutch word for cloth, doek.

[*] Denim is a sturdy cotton warp-faced textile in which the weft passes under two or more warp threads. This twill weaving produces a diagonal ribbing that distinguishes it from cotton duck.

[*] Linen is a textile made from the fibers of the flax plant. Linen is laborious to manufacture, but the fiber is very absorbent and garments made of linen are valued for their exceptional coolness and freshness in hot weather.