In reality, there are no holes! It just refers to positive charge.
The diagram below shows positive charge travels from the left to the right of silicon. At t1, one electron is released which produces a positive charge (indicated as a hole); at t2, the second electron is released and trapped in the hole shown in t1; at t3, the third electron is released and again trapped in the hole shown in t2.
This explains why movement of holes is shower than that of electrons.
For pure silicon, the density of holes is equal to the density of electrons (ni).