In conductors, electrons are loosely bound to the nucleus hence, can detach easily at room temperature. Also, a large number of free electrons thus, available are conduction electrons. When the covalent bond breaks, electrons are freed from the atom. The departure of an electron from valence band creates the vacancy in bond, this vacancy is known as hole. This hole is captured by another free electron. Also, the energy level of free electrons corresponds to the conduction level, hence valance level and conduction level are overlapped. So there are no holes in conduction level to carry hole current.