What is the difference between Kuwa na krismasi njema and Heri ya krismas?

No real difference except that Kuwa na Krismasi njema is exceptionally bad Swahili. It's intended to mean "Have a nice Christmas," but kuwa na is wrong: would be Uwe na, etc. However, even uwe na would never be used; the phrase is not idiomatic Swahil.In standard Swahili, Krismasi njema and Heri ya Krismasi are both commonly used as Christmas greetings and mean exactly the same thing: Christmas blessings (Happy Christmas, Merry Christmas).