They are bluffing and saber-rattling.
The Serbs have placed their hopes in a megaphone, not an ally.
The Russians lost 8,000 soldiers in 20 months in Chechnya fighting a population half the size and far less than half as well-equipped as Kosovo, with no support from NATO.
i agree, the risk of direct conflict is limited.
That being said, if Russia were going to fight a conventional war with NATO - this would be as good as it gets for them. This is their backyard and the logistics train would be much shorter for them, than us.
Putin does have some leverage here and could easily enough move some airborne troops into Serbia. I doubt he has the gumption to try to insert them directly into Pristina, although Yeltsin pulled that off back at the end of the Kosovo bombing campaign.