Rashi: It is south of the Pelishtim. We find elsewhere that Pelishtim are called Goy Keresim - "Yoshvei Chavel ha'Yam Goy Keresim" (Tzefanyah 2:5).

Radak: It is [south of] a family of Pelishtim, like it says "Goy Keresim" (Tzefanyah 2:5), "v'Hichrati Es ha'Keresim" (Yechezkel 25:26).They spread in Eretz Pelishtim and Eretz Yehudah, like it says "Asher Lakchu me'Eretz Pelishtim ume'Eretz Yehudah" (verse 16).

Radak: Kalev is Kalev ben Chetzron, from Shevet Yehudah. Even though it says "v'Al Asher li'Yhudah", Kalev's inheritance had its own name. Negev means south, or a plain. It is called Negev, just like a Midbar is called Negev, due to the desolation, and because it is flat.