Value of CVA is marked with positive prefix as this is considered as a cost.
Value of DVA is usually marked with negative prefix as a cost deduction.
Thus, stand-alone CVA cannot be negative while net BCVA can be negative in the event of larger DVA value.
Hope that might help.
As per Gregory chapter 14 he has introduced a negative sign in CVA formula, so am a little confused as to what should be the impact of credit spread and recovery on CVA. Whether we have to consider the magnitude or sign also