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 (unofficial) and the current Daventry.
Note: Daventria may also be name of the world (but 'earth' is also used in KQ1 by AGDI)