Given the first part of Sylveon's name, my guess is that it's a Metal type. With every other Eeveeloution, the part of the name preceding "eon" has always indicated the Pokemon's type.
I've heard that before; people see "Sylv" and think "Silver".
Ignoring how the Pokémon looks (I've already said, while a stretch it could be justified as a Metal-Type despite the looks), consider the Pokémon's name in other languages.
Spanish: Sylveon
"Silver" is "plata" in Spanish.
Italian: Sylveon
"Silver" is "argento" in Italian.
French: Nymphali
"Silver" is "argent" in French
German: Feelinara
"Silver" is "silber" in German.
Of course, if that wasn't what you mean, my bad. That is how it has been explained by others who have suggested it.
"Fee" means "fairy" in German and "nymph" (or nymphe) means "nymph" in French, if Google translate can be trusted. Both are magical creatures that may be spirit instead of flesh. "Silvan" relates to wood spirits and sometimes magical woodland creatures, and is derived from Silvia/Sylvia, Roman goddess of the woods and the moon.
I have also heard people suggest that the "Sylv-" comes from "Sylph", a term used for certain spirits and in particular wind spirits (or "magic creatures", depending on the setting), but this would require the "ph" be transmuted to a "v"; such a change has not occurred with any other English name for an Eeveelution, but it does occur with other Pokémon so this could be the first.