It goes something like this....
1979
# KDX400-A1, first KDX model introduced
1980
# KDX80-A1 introduced
# KDX175-A1 introduced
# KDX250-A1 introduced
# KDX400-A2
1981
# KDX80-A2
# KDX175-A2
# KDX250-B1
# KDX420-B1, KDX400 goes to KDX420
1982
# KDX80-B2
# KDX175-A3, last year for KDX175
# KDX250-B2
# KDX450-A1, KDX420 goes to KDX450, last year for KDX400/420/450
1983
# KDX80-B3
# KDX200-A1, first year for KDX200
# KDX250-B3
1984
# KDX80-C1
# KDX200-A2
# KDX250-B4, last year for the air-cooled KDX250
1985
# KDX80-C2
# KDX200-A3
1986
# KDX80-C3
# KDX200-C1, front disc brake, power valve engine, 43mm conventional forks
1987
# KDX80-C4
# KDX200-C2
1988
# KDX80-C5, last year for the KDX80
# KDX200-C3, available with lime green frame and lime green plastic or silver frame and white plastic, last year for air cooled full sized KDX
1989
# KDX200-E1, heavily redesigned with liquid cooled engine, rear disc brake, different frame, improved suspension, etc.
1990
# KDX200-E2, available with lime green frame and plastic or sky blue frame and white plastic
1991
# KDX200-E3, last year for KDX200 available with lime green frame and plastic or sky blue frame and white plastic
# KDX250-D1, KDX250 reintroduced with perimeter frame, liquid cooled engine, USD forks, rear disc brake, etc.
1992
# KDX200-E4, last year for full sized KDX with damping rod forks
# KDX250-D2
1993
# KDX200-E5, KDX200 receives upside down (USD) forks, 41mm inner tubes
# KDX250-D3, change to full floater front disc rotor
1994
# KDX200-E6
# KDX250-D4, last year for KDX250
1995
# KDX200-H1, KDX200 totally redesigned with perimeter frame, conventional forks, engine, brakes, shock, etc.
1996
# KDX200-H2, various parts in violet (purple)
1997
# KDX200-H3
# KDX220-A4, KDX220R released, almost entirely the same as KDX200, engine bore size larger, o-ring chain
1998
# KDX200-H4
# KDX220-A5
1999
# KDX200-H5
# KDX220-A6
2000
# KDX200-H6
# KDX220-A7
2001
# KDX200-H7
# KDX220-A8
2002
# KDX200-H8
# KDX220-A9
2003
# KDX50-A1, KDX50 released (as part of Kawasaki - Suzuki alliance)
# KDX200-H9, change from lime green frame to gray/silver frame
# KDX220-A10, change from lime green frame to gray/silver frame
2004
# KDX50-A2
# KDX200-H10
# KDX220-A11
2005
# KDX50-A3
# KDX200-H11, frame color changed to silver, lighter lime green plastic
# KDX220-A12, frame color changed to silver, lighter lime green plastic, last year for KDX220R
2006, last year for all KDX's :-(
# KDX50-A6F
# KDX200-H12
credit to KDXGarage for compiling this info,
http://www.kdxgarage.com