Help with rear shock choice

Still trying to decide on a replacement rear shock for my Specialized 2009 FSR XC. I found a Fox RP2 that is a XV. What exactly is a XV shock and what difference does it make to the ride over the standard can? I'm also not sure of which tune I need to get for my bike. The one I found is a medium rebound and compression and 175 psi boost valve pressure. Is that suitable? What significance is the boost valve pressure?

Thanks