The CR-V uses front wheel drive normally, with a hydraulic mechanism to provide instant power to the rear wheels when there is slippage. Google has info on this system.
Subaru does not use just one system. There are several. The MT often use a viscous clutch that takes a quarter wheel spin to provide power. The 4EAT uses clutches that run 90/10 front to back normally and provide variable degrees of power to the rear wheels on slippage. The on-board computer monitors wheel slippage and adjusts lock up on the fly.
As for needing winter tires, a 4WD system needs them MORE than two axle drive systems for safe driving in icy weather. Many people endanger others' lives by not fitting winter tires, and in many jurisdictions that is allowed. In other places, the couple of weeks of cool weather do not necessitate winter tires at all.