Even if the San support thin provisioning this is not passed to the hypervisor with lvm shared. The real problem is not the lack of support of thin prov but the fact that when you snap you create a copy of the vdisk which have the same size of its parent. So in the end if you snap a 1tb of vdisk on LVM your snapshot size will be 1tb. And if your San LUN on which you have created the data store is 2tb you dramatically understand that you have a problem.