I have yet to meet a Volibear that can beat Sivir.
Volibear is a burst champion that does misleading amounts of damage. What I currently see is a burst tanky champion that wants to be a DPS, and manages to fool everyone about his damage output because his W allows him to KS so easily. So people (and Volibear players) might see Volibear going 11 kills and 3 deaths but they don't realize that it isn't Volibear that's actually the threat on the team, but actually the Kog'Maw who never got any of the last hits because of that W.
It's the same thing with Garen. Garen might be going 10 kills ahead but that's no reason to focus him at all. At no point in the game would it be wise to focus Garen regardless of how many kills he stole rightfully obtained through clever tactical planning. That's because Garen is the same, a burst, tanky champion that does misleading damage allowing him to killsteal easily. By the time you kill him, he's already used his combo and you wasted all that time and effort. What's the point?
Yes yes I know, he has two skills that make it seem like he's capable of DPS. His ult and his passive W. But these aren't exactly super high damage skills. Practically speaking, they output the same level of threat as Singed's poison trail. It's significant, but not significant enough to warrant you chasing Singed around the map to end his "EPIC DEEPS".
So, tl;dr: Treat him like Garen. Ignore and kill last. Use positioning, kiting, heals, CC, and lifesteal to ruin his day.