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Why can't I do VGA passthrough to Xen PV (paravirtual) guest?
Xen PV domUs are lightweight by design so they don't have legacy x86 features like BIOS and various other features. This means complicated x86-centric features like VGA emulation are not easy for Xen PV guests. Note that some graphics cards might work in the PV domU just by using the normal Xen PCI passthru.
Can I share the graphics card between multiple VMs with Xen VGA passthru?
In short, no. Xen VGA graphics passthru is a special form of PCI passthru, and PCI passthru dedicates the PCI device (graphics card) to exactly one single VM.