Konbanwa spells こんばんは in Japanese Hiragana letters. Please note that the correct spelling is こんばんは, not こんばんわ. It may be a bit confusing, but the last WA sound uses “は” in Ha Hi Fu He Ho. See the image below and check the correct writing.
As for the spelling of Konbanwa, Even Japanese (especially little kids) sometimes make mistakes. To tell you the truth, こんばんわ has been considered the correct spelling in Hiragana until 1986. The Japanese government defined how to write Konnichiwa and Konbanha in Hiragana in 1986. Therefore some elder people still write Konbanwa こんばんわ. But it’s clear that こんばんは is the correct way in the current days!