What are holes? How they exist in pn junction
A hole exists where an electron should be... but isn't.

It's the absence of an electron.


  In physics, chemistry, and electronic engineering, an electron hole is the lack of an electron at a position where one could exist in an atom or atomic lattice. It is different from the positron, which is an actual particle of antimatter.
  
  -- Wikipedia


When you dope the semiconductors for a P-N junction you add a dopant to the N portion which has an excess of electrons (more than would normally be there) and you add a dopant to the P portion which has a lack of electrons (less than would normally be there).

The electrons would be able to flow from the N to the P so they can occupy some of the holes in the P portion.


  After joining p-type and n-type semiconductors, electrons from the n region near the p–n interface tend to diffuse into the p region. As electrons diffuse, they leave positively charged ions (donors) in the n region. Likewise, holes from the p-type region near the p–n interface begin to diffuse into the n-type region, leaving fixed ions (acceptors) with negative charge. The regions nearby the p–n interfaces lose their neutrality and become charged, forming the space charge region or depletion layer
  
  -- Wikipedia