I found the Rules and Game Systems - Official Rules section of the Tékumel Official Site and History of the Empire of the Petal Throne RPG, by Brett Slocum, useful in preparing the following. All quotes taken from History of the Empire of the Petal Throne RPG, except where noted.
1975 - Empire of the Petal Throne (EPT) has "a relatively simplistic rules set based on Dungeons & Dragons" - Official Rules - Empire of the Petal Throne
1983 - Gamescience reprints the original EPT and Swords and Glory Volume I: "a complete description of the world with no rules."
1984 - Gamescience prints Swords and Glory Volume II: "Player's Handbook, giving the character generation, combat and magic rules (completely new mechanics, plus an 8-page character sheet)." This edition was "incomplete" having no Referee's Handbook because in...
1989 - Gamescience gave up on ETP when "Hurricane Hugo [destroyed] the galleys [for Volume III: Referee's Handbook] and all remaining warehouse copies of Volume II."
1992-1994 - Theater of the Mind Enterprises published "the Adventures on Tékumel series, including Tékumel Bestiary, character generation... and Gardásiyal, the rest of the game rules". Disappointingly, "[t]he Adventures on Tékumel books [are] required for source material such as new characters." - Official Rules - Gardásiyal: Deeds of Glory
2005 - Guardians of Order publish a "a complete game in one book," "based on the Tri-Stat system of BESM." For more on the Tri-Stat dX see Wikipedia.
Brett Slocum also appears to have links to the majority of the rules at RPG Rules for Tékumel, where you may be able to pick out more detailed differences between the editions. Finally, both Brett and Rules and Game Systems - Unofficial Rules have alternate rules and links to conversions in other systems.