Energy has to be supplied to move electrons away from the nucleus of the atom. The valence electrons have the highest energy levels of the electrons that are still bound to their parent atoms, (as they are furthest from the nucleus). Additional energy is required to completely remove an electron from the atom, so free electrons have higher energy levels than valence electrons. (Free electrons are the mobile charge carriers that enable metals to conduct electricity.) This can be illustrated with an energy band diagram, which shows two energy levels, a valence band and a conduction band. Valence electrons are located in the valence band and the free electrons in the higher conduction band.
In semiconductors there is a gap between the valence and conduction bands. So energy must be supplied for valence electrons to "jump up" to the conduction band. This reflects the fact that energy must be supplied to remove valence electrons from their parent atoms and become free electrons. In insulators this gap is much larger, to represent the significantly higher energy levels that would be needed, to "pull" electrons from their parent atoms. In metals the valence band and conduction band actually overlap. So in metals, valence electrons can move easily into the conduction band, producing a large number density of free electrons.
When pure silicon is at a temperature of 0K, all of the positions in the outer electron shells are occupied, due to the covalent bonds between atoms and there are no free electrons. Therefore the valence band is completely full and the conduction band is completely empty.