Help understanding Keychain passwords vs Safari passwords
Like Tik27 said, when Safari saves a password it is saving it to the Mac Keychain. There is no way for Safari to actually save a password, it hands it off to the keychain.

So any time Safari (or Mail.app, Finder, etc.) offers to save a password it is being saved in the keychain.

you can verify this by signing on to a new website with a username and password, letting Safari prompt you to save the password and then opening Keychain.app and seeing the new username/password saved there with a date/time stamp of only minutes ago.