While the spelling and sounding of the words “Lithuanians” and “Litvinians” are similar, these two concepts mean different phenomena. Lithuanians are called ethnos, which formed over many centuries from two tribal kinship groups. The first one is the Aukštaits, the Baltic tribes that inhabited the eastern region and the center of modern Lithuania. The second group of tribes - the Samogites, or Zhmudins - lived in its western part.
Aukštaity and Zhmudiny
From the Aukstaits and Zhmudins, a single ethnos was gradually formed, and then a nation with a common language, culture and statehood. This nation is now called Lithuanians. Ethnic Lithuanians can live anywhere - in Australia, Siberia or Canada - and remain Lithuanian. This concept means nationality, common roots.
Who are the Lithuanians
Litvin is more of a "territorial" term. It denotes the entire population of the Grand Duchy of Lithuania (XIII-XVIII centuries). This once powerful principality occupied a vast territory. It included many lands: all of modern Belarus, a large part of Lithuania and Ukraine, some regions of Latvia and Estonia, part of Poland and Moldavia and the entire west of Russia.
Thus, the Lithuanians were not only ethnic Lithuanians proper, but also all Belarusians, some of the Ukrainians, Moldovans, Poles and representatives of other ethnic groups who lived on the listed lands. In order to be considered a Lithuanian, one did not need to belong to the Lithuanian nation at all.
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The word "Litvin" was often used as an opposition between the Western Slavs living in the Grand Duchy of Lithuania and the Eastern Slavs (“Rusyns”) who lived in the Grand Duchy of Moscow. These 2 powerful states were strongly at enmity in the XV-XVI centuries. Serious territorial claims were the cause of the enmity. The names “Rusyns” and “Litvins” were supposed to further emphasize the difference between the population of these principalities.
Modern history
It is interesting that in the 21st century a whole movement has emerged with the goal of reviving the former meaning of the concept of "litvin". The disappeared Grand Duchy of Lithuania was renamed by the representatives of this trend in a new manner in “Great LITHUANIA” (GREAT LITHUANIA). She is considered the heiress of a long-defunct principality.
Any Belarusian, Pole, Ukrainian, Moldovan, Estonian, Latvian, or ethnic Lithuanian living now on the territory of Great Litvin (these are the historical regions that were part of the Grand Duchy of Lithuania many centuries ago) can rightfully be called Litvin. Representatives of this movement believe that the Lithuanians are an open nation, accepting into their friendly family any person who feels the need to join the culture of this people.