FVM solves the equation in conservative form. Or in other words conservative form is derived using continuity, therefore the scheme is positively conservative always. It is localy conservative. (Finite Volume Methods: Robert Eymard, Thierry Gallou ̈et

and Rapha`ele Herbin).

I am also learning CFD from last two year. So, in what context you mean, it two to be positively preserving.

The sum of all finite-difference equations coefficients divided by the central coefficient to be (in absolute terms) less or equal to 1 and less than 1 at one at least point. This is a necessary but not sufficient condition. There is none for both