SSPX, SSPV, Sedevacantist "independent." Is there any real difference among these groups?
Meaningful to them, less so to us.
Yes. The SSPX, while in, at minimum, an irregular relationship with Rome, claim to recognize the Pope as valid. They also are quite possibly on the verge of being regularized.
The others do not and are not.
It is true, though, that there are those within the SSPX (which is a society of priests), and some laymen who assist at SSPX chapels who are functional sedevacantists. What proportion are like this, I cannot say.
There have been numerous high level contacts between Rome and the SSPX over the last 5-10 years or more and now evidently the Holy Father very much wants a solution with SSPX. I have never seen the others mentioned ever in this way.
On the practical level at least, is that enough of a difference?
Yes, a bit of difference, in terms of the missals they celebrate, all the sdes celebrate the pre 1955 missal, do the fact the SSPX at least nomincally recognise the Pope as the real Pope. Also on the issue of No Slavtion outside the Cathiolic church(EENS), the SSPX has a considerably looser definition.