That is because NV is convex. Therefore it is not allowed to be on the RHS of <= constraint.
http://cvxr.com/cvx/doc/basics.html#constraints
The following constraint types are supported in CVX: Equality == constraints, where both the left- and right-hand sides are affine expressions. Less-than <= inequality constraints, where the left-hand expression is convex, and the right-hand expression is concave. Greater-than >= constraints, where the left-hand expression is concave, and the right-hand expression is convex.