Several long latency operations are not preemptible
Page table manipulation operations for PV guests can take significant amounts of time, as they require all present branches to have their type (and thus contents) verified. While the most frequently used operations had been made preemptible in the past, some code paths involving potentially deep page table traversal were still trying to do their entire work in a single step.
long operations not preemptible
There are many patches to make operations preemptible, see here
Malicious or buggy PV guest kernels can mount a denial of service attack affecting the whole system.