An insulator has a large energy band gap between the conduction band and valence band. There will be no carriers in the conduction band at 0 K as well as at 300 K.

A semiconductor has a moderate energy band gap and there will be no carriers in the conduction band at 0 K. There will be carriers in conduction band at room temperature.

A conductor has overlapped conduction and valence band (energy band gap do not exist). The conduction band is partially filled with carriers at 0 K and it will be fully filled at room temperature. Therefore, the material is a conductor in which the conduction band is partially filled at 0 K.