Is there a difference between the par value of a bond and its principal?

To my knowledge, principal is usually not used to refer to a bond. This is probably because the price of bond change, varying redemption value exists for some bonds, and the outstanding balance (principal left unpaid) change in case of an amortized bond. Par is the amount printed on the bond.

Actually principal is often used to refer to the amortized amount. So if you have a bond that doesn't amortize, Par=Principal, the principal is always the same as the original face value of the bond. But for bonds that can be paid down: Par stays the same (original face value), principal can be paid down (current face value becomes lower than par). When you carry an amortizing bond (like a capitalized lease, or a car payment) on financial statements, you show its current value as principal and interest owed. This is NOT the same as the present value of the bond which is (price*.01*principal) +accrued interest.