I've upgraded from XenServer 6.2 to 7.0 and also upgraded PVS servers to 7.9 (via 7.8 for a day or two).
Existing vDisks (PVS 7.1 target device software, XS 6.2 tools) and updated vDisks (PVS 7.8 target software, XS 7.0 tools) both take around 18 mins to boot hanging on the "Starting Windows" screen. Some VMs get stuck and run at 100%, they never successfully boot. Once booted, the vDisk status shows a boot time of around 22 secs and a low uptime. Retries are negligible (0-10). I'm assured (though didn't test it myself) that prior to the XenServer and PVS upgrades, the boot times were nowhere near that high.
I've done netio speed tests between VMs, hypervisors and PVS servers; all were decent.
I've tried rolling back to 7.1 bootstrap.
I've tried creating a new vDisk with a clean Windows installation.
I've tried removing 'ghost' network devices in Device Manager.
Large Send Offload is disabled both client-side and server-side (and on the hypervisors).
I've tried booting from a virtual PVS server on the same hypervisor as a VM.
No logs in either client or PVS server event viewer.
PVS servers have 6 NICs (Broadcom) teamed with 2012R2 Teaming in switch-independent mode