At Nissan, SV apparently is internal code for "one or two steps above the base model." The SV models in Nissan's lineup are a step up from the S version in price and standard equipment, and the S models are typically the lowest or second-lowest priced versions.
You can see how this plays out with the redesigned 2013 Nissan Altima, which comes in base 2.5 (denoting the engine size in liters), S, SV and SL trim (or price) levels.
Other Nissan models follow a similar pattern, though there are differences. For example, the Pathfinder and Murano come in S, SV and SL trim levels; the Pathfinder adds a top-shelf Platinum model; and the Murano has an LE version as its most-expensive offering.
Automakers use a variety of model designations to denote plain or fancy versions, and there's no rhyme or reason behind the letters and numbers. One company's S model, for example, may be a direct rival for another's SE, LE or LX model. They're just another way of saying Good, Better, Best.