Preface: Xen Project is a hypervisor using a microkernel design, providing services that allow multiple computer operating systems to execute on the same computer hardware concurrently.
The vulnerability is due to an interpretation conflict for union data structure associated with shadow paging.The XSA-240 introduced a new field into the control structure
associated with each page of RAM. This field was added to a union data structure.Thus dirty bitmap tracking which is used when performing live migration of virtual machines. However a technical conflicts during migration, or L1TF mitigation for PV guests(L1 Terminal Fault speculative side channel mitigation – XSA-273).
Impact: All Xen versions from at least 3.2 onwards are vulnerable. Earlier versions have not been checked
Official remedy solution: https://xenbits.xen.org/xsa/advisory-280.html