While more can always be added, /u/textandtrowel's answer to the question "Origins of the word "slave"" may contain the information you are after.

To add, according to the Oxford English Dictionary, the two words derive from the same Latin word, with "Slav" being the original and "slave" being the derivative. But they may have taken slightly different paths into the English language. "Slave" was a borrowing from Old French after the Norman Invasion, and Old French had derived it earlier from the original Latin. "Slav," on the other hand, may have come directly from Medieval Latin, which English encountered via the Roman Catholic church.