I worked with a Russian boy with this exact name & spelling. Since everyone generally stumbled over the pronunciation, he went by the nickname of "Sevie."
I love this name. I'm Russian, and for me this name is the strongest and most powerful Russian name. It sounds monolithic, if I can use this word, it sounds wholesome, solid, so I would never call a person who bears this name by any diminutive. And yes, it is very rare, unique.
In Fyodor Dostoyevsky's 'The Possessed' or 'Demons' there is a character Nikolai Vsevolodovich Stavrogin (the central character in fact), whose father's name is Vsevolod.
This name belongs to my boyfriend, so it makes me pretty happy. : ) He says that it is rare these days (he has never met another Vsevolod, even in Russia) but there were several princes with this name in the Middle Ages. The correct Russian diminutive is Sevka (approx. SEV-ka or maybe SYEV-ka), but English-speakers call him Sev.