Books have been written on this subject. FWIW, "Kingsville" has become a loose term for any number of small-leaved boxwood types. The one I have pictured above is a "true" Kingsville. I bought it from a source who has cuttings from the original trees found in the Kingsville, Md. nursery that gave the cultivar its name. The seller regularly brought older Kingsville to the National Arb show and sale every year, but she no longer is able to, from what I hear.
From what I've seen American and English boxwood (buxus sempervirens and buxus sempervirens "suffruticosa" ) are not all that common as bonsai subjects, although they probably should be. The typical bonsai boxwood is Japanese boxwood--Buxus Microphylla-- Winter Gem, Morris Midget and a few others. Korean boxwood Buxus Koreana is a close second. Korean box, with its quickly weathered tan bark, makes great bonsai that can withstand extreme cold. I had a few old trees for a very long time, but they were lost to soggy soil over one winter., unfortunately.
http://www.boxwoodsociety.org/Handbook/Handbook.htm
http://saundersbrothers.com/index.cfm/fuseaction/plants.main/typeID/16/index.htm