Why absence of electron is called hole? I am having hard time in understanding the concept of holes:


*

*If there is no electron than how can it be a hole?

*For a moment lets assume absence of electron is termed as hole but how can this absent particle have mass?

*Under equilibrium  no electrons are present in conduction band. Why cant we termed this absence as holes in conduction band?
 A: In an idealized semiconductor at absolute zero, all the valence states are occupied by electrons, and all the conduction states are empty.  When you take one electron an place it into the conduction band, you leave behind a state that is no longer occupied in the valence band. 
Now, lets say you have a sample with a billion electons, all in the valence band, and you pop one into the conduction band.  Figuring out how that one electron can move about in the conduction band is easy - there isn't anything else there to worry about.  For the valence band, you could now worry about how the 999,999,999 electrons move, or you could instead say, wow, there is one empty state in a sea of full states - wouldn't it be easier to figure out how the empty state moves and focus on just that one?
This is a common technique in physics, be it called renormalization, or quasi-particles, or whatnot - reframe the problem to make it simpler. 
So, the "hole" is what we call the empty state - there is an electron "missing" in the valence band, and we watch it move around. To avoid saying 'the state not occupied by an electron' lots of times, it got shortened to 'hole'.
A: *

*The absence of a material in a volume embedded into a larger volume that is fully filled by the material is always known as the hole. For example, look at this hole in soil. In semiconductors, the "soil" is replaced by the semiconductor material itself, with the right number of electrons per nucleus to make it neutral. So if there is an electron missing relatively to the expectations, it is a positively charged hole. The positive charge "microscopically" comes from the nuclear protons that are not "cancelled" but we are assuming that the nuclei together with all the electrons behave like a simple environment, like soil or the vacuum, so we may measure charges relatively to that.

*Holes have (positive) mass and many other features analogous to the electrons themselves because there exists (at least qualitatively) a symmetry that replaces all occupied electron states by unoccupied, or vice versa. For fermions, the occupation numbers are only $N=1$ or $N=0$ for a given state, and $N\leftrightarrow 1-N$ maps this set of possibilities onto itself. That's why the exchange of electrons with holes must keep the formalism pretty much unchanged. A careful scrutiny of signs implies that the holes have a positive inertial mass, the $m$ from the kinetic energy term $p^2/2m$. That's effectively because holes like to exist with momenta $p$ near the maxima of a function of $p$ (this momentum parameterizes states) – the opposite than electrons – so one would get a negative mass but this sign is changed once again because we're talking about holes i.e. absence of electrons.

*The states in the empty conduction band aren't called "holes" because the "fully filled state" (recall the soil analogy) isn't allowed. By definition, the conduction band is empty at rest. If there's no soil, there can't be a hole in the soil, either.
