In metals, the conduction band is either partially filled or the valence band is partially empty. There are electrons that behave as free electrons as they shift to higher energy levels by acquiring energy above the Fermi level in the conduction band. There is no forbidden energy gap in the metals. Since there is no forbidden gap, the number of electrons available for the conduction is more increasing the conductivity of the material. Metals behave as a conductor because of the movement of the free electrons when a small amount of electric current is applied.
In insulators, the valence band is completely filled while the conduction band is empty. This results in a large energy gap. Since the energy gap between the conduction band and the valence band is more, there is no movement of electrons from the valence band to the conduction band.
In a semiconductor, the valence band is completely filled with electrons while the conduction band is empty. The energy gap between the bands is less. For electrons to jump from the valence band to the conduction band, room temperature needs to be maintained. If the temperature is 0K, there is no transfer of electrons from the valence band to the conduction band.