I too am in the same boat with my new 2015.5 XC70 with 18" wheels.
XC70 18" summer tires are 235/50/18 , which have a diameter of 27.25"
XC70 17" winter tires are 235/55/17 , which have a diameter of 27.16"
XC90 17" winter tires are 235/65/17 , which have a diameter of 29.0"
So with an XC90 17" winter setup, your XC70 will sit 7/8" higher off the ground -> ( 29.0" - 27.25" ) / 2
... and protrude 7/8" more all around filling your wheel well
I can see a couple of issues with using XC90 wheels and tyres on a P3 XC70.
Firstly have you checked the XC90 wheel will fit the hub on the XC70? I do vaguely recall that the 1st gen XC90, P2 and P3 Volvos all share the same bolt circle and bolt details although there may be differences in hub diameter - although logic suggests that the wheel from an XC90 probably has a larger hub diameter than a P3 Volvo so you should be OK. You should check hub/bolt details between an XC90 and P3 XC70 (if you haven't already done this).
I don't know the specific answer to this but I would strongly suggest the 1.8" larger diameter of the XC90 tyre will make contact with bodywork particularly when the front wheels are turned hard over. Certainly I would think you wouldn't be able to fit snow chains.
My final thought is the impact the larger diameter tyre will have on your speedo (and odometer). The 29.0" vs ~27.2" overall diameter would result in your speedo reading 6-7% low - ie at an indicated speed of 60mph you are actually doing 64 mph. Certainly here in Australia that would be illegal (we are limited to a maximum variance in overall tyre diameter from the OEM size of -26mm/+15mm or -1in/+0.6in). You might need to check your local laws.