So, I want to know the detail working of Flash Drive(Pen
Drive) i.e when we insert/remove it into/from a particular port, how
drivers get loaded/unloaded.

Read:

a) USB spec, including the USB Storage spec
b) MS's documentation on WDM and PnP

This cannot be explained in a few words.

--
Maxim S. Shatskih
Windows DDK MVP

http://www.storagecraft.com