Exchange or Microsoft SQL Server) flush data to disk and prepare for the snapshot before it is taken.
Quiesced snapshots are therefore safer to restore, but can have a greater performance impact on a system while they are being taken. They may also fail under load so more than one attempt to take the snapshot may be required.
XenServer supports quiesced snapshots on:
• Windows Server 2012 R2 Server Core
• Windows Server 2012 R2
• Windows Server 2012
• Windows Server 2008 R2
• Windows Server 2008 (32/64-bit)
• Windows Server 2003 (32/64-bit)
Windows 8,1, Windows 8, Windows 7, Windows 2000, and Windows Vista are not supported
Note : VSS and quiesced snapshots have been deprecated starting Citrix Hypervisor 8.1
https://docs.citrix.com/en-us/citrix-hypervisor/whats-new/removed-features.html