Par value is a value for the bond or for the stock beyond which a bond or a security in general can be deemed mature. For corporate-issued securities this is a price of $1000 and higher. For government bonds the amount has to be bigger. Securities can be traded both above and beyond par value.
Per diem interest is the daily interest on a loan that comes up outside of the standard repayment period. These charges may occur if a borrower receives their principal payment and begins the loan repayment period on any other day except for the first day of the month.