The energy band scheme of semiconductors is shown here.

In semiconductors, valence band and conduction band are separated by an energy gap called the forbidden energy gap. It is very small. At room temperature some electrons in valence band acquire thermal energy. This energy is more than forbidden energy gap , thus they jump into the conduction band and leave their vacancy in the valence band which act as holes. Hence, at room temperature valence band is partially empty and conduction band is partially filled.