Is Early Voting constitutional?

[Verily]

Can someone explain the difference between "Early" voting & "Absentee" voting?

I have voted absentee twice due to knowing in advance that I would not be able to get to the polls because of travel, so in essence I voted early. Maybe I am missing something here.

Absentee ballots are not opened (and thus the vote has technically not yet occurred) until the day of the election. Early voting is a practice where polling stations open on days before the election to allow voters to cast ballots then for convenience. I've only heard of it in the context of Texas, but maybe other states practice the same thing.

I'm supportive of early voting, and it appears to be constitutional (it's the electors who are bound to a single day). However, I prefer the idea of online voting, recently introduced by Estonia to great success. Obviously the server would need to be secure and provide a printed paper trail at its destination, but allowing voting online would greatly increase turnout.

[Verily]

As long as the ballots are counted on election day, I don't see anything wrong with it.

Unlike absentee voting, which I believe should be illegal except in certain excemptional situations.

That seems like a wonderful way to disenfranchise college students, for example, who usually go to school far enough away from home (4+ hours) that it isn't feasible for them to return home to vote. If Election Day were in April, maybe, but with Election Day in November, there's not enough time to register locally.

I'm assuming you consider the military "exceptional".