Is it correct that the potential difference in closed loop is zero? Why??
Yes, sort of.
The correct statement of the loop rule for electric circuit analysis is: "The sum of all the potential differences around a closed loop equals zero."
This is really a statement of a more fundamental conservation rule. We might call this rule "conservation of current." If current flows into some point it must also flow out of that point.
Here is a great reference describing Kirchoff's Loop Rule:
Kirchoff's Loop Rule