What is the difference between revision and novation?

Hello,
I recently got help from my bank on my mortgage loan as they were willing to do a modification. However, i was originally in a negative amortization loan and now to help us keep our house, the bank is willing to help with a 18 month interest only deal. The wording on the signature part said that ” this Agreement is only a revision and not a novation. please help explain what that means in legal term. and my obligations thereafter the 18 months,. i’m most concern about it going back to a negative amortization situation after the 18 months. thank you for your time.

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