Is there a risk of a VM reading memory from a previous VM?

Q. Is there a risk of stopping a Hyper-V VM and starting another one which could then read the memory pages from the other VM?

A. No. Hyper-V utilizes strict isolation between VM instances. When a VM is stopped before another VM is allocated the same memory pages that memory is zeroed before being reallocated. It is therefore impossible for a VM to read residual page content that were populated by a different VM.