Nothing, really. Kudos comes from the Greek ?????, meaning “glory,” "renown" or “fame.” The Greek “-os” ending typically indicates a singular noun, so kudos doesn’t signify more than one kudo, but a single thing. The ending is supposed to be pronounced like “-ose”, rather than “-oze” which is how many Americans say it; this contributes to the singular/plural confusion.
While it might not be right, kudo is at least acceptable by the standards of some dictionaries. It's been around in print since at least 1926—enough time to get itself considered a valid word, meaning the same thing as kudos, through usage.