The Kalos Universe is the shared fictional universe where most comic book titles published by Kalos Comics take place, including those featuring Kalos' most familiar characters, such as Paragon, Swan, and Manticore.
The Kalos Universe is further depicted as existing within a "multiverse" consisting of a large number of separate universes, all of which are the creations of Kalos Comics and all of which are, in a sense, "Kalos universes". In this context, "Kalos Universe" is taken to refer to the mainstream Kalos continuity, which is known as Earth Zero or Earth-0.
The universe of Earth-0 has no particular significance in comparison to that of Earth-1 or any other universe -- the name is simply a convenience. A more technically accurate nomenclature would define the "Kalos Universe" in terms of its "location" in the multiverse within 196,883 dimensional space. Since this would require the listing of 196,883 coordinates, "Earth-0", "Earth-162", "Earth-4661", etc., is a convenient shorthand.
(The Mainstream Universe does not exist within this framework. The spaciotemporal relationship between the Kalos Universe and the Mainstream Universe is not yet understood.)