The information in this article is from the fan game King's Quest III Redux and is therefore not canon.
Daventria was a large continent and nation founded by the First King, Legenimor.
Due to an upheaval it was split into smaller continents and islands including Treasure Island and the current Daventry.
Note: Daventria may also be name of the world (but 'earth' is also used in KQ1 by AGDI)