In relation to a person it is defined as the unexplainable element of a person's attractiveness or sexiness. This is more of a slang and is not a part of the English Language (as in being accepted into the Dictionary) yet.
And the x factor is that something extra that you have that the other person does not.
X factor is finding hidden beauty in an un-appealing personality and enhancing small factor to cover remaining deformities.
Is karan got any of these?? certainly does, that is why he got many fans..