Xen Project 4.2.1 is a maintenance release in the 4.2 series and contains: We recommend that all users of Xen Project 4.2.0 upgrade to Xen Project 4.2.1.

The release fixes the following critical vulnerabilities:

CVE-2012-4535 / XSA-20: Timer overflow DoS vulnerability
CVE-2012-4537 / XSA-22: Memory mapping failure DoS vulnerability
CVE-2012-4538 / XSA-23: Unhooking empty PAE entries DoS vulnerability
CVE-2012-4539 / XSA-24: Grant table hypercall infinite loop DoS vulnerability
CVE-2012-4544, CVE-2012-2625 / XSA-25: Xen domain builder Out-of-memory due to malicious kernel/ramdisk
CVE-2012-5510 / XSA-26: Grant table version switch list corruption vulnerability
CVE-2012-5511 / XSA-27: Several HVM operations do not validate the range of their inputs
CVE-2012-5513 / XSA-29: XENMEM_exchange may overwrite hypervisor memory
CVE-2012-5514 / XSA-30: Broken error handling in guest_physmap_mark_populate_on_demand()
CVE-2012-5515 / XSA-31: Several memory hypercall operations allow invalid extent order values
CVE-2012-5525 / XSA-32: several hypercalls do not validate input GFNs

Among many bug fixes and improvements (around 100 since Xen Project 4.2.0):

A fix for a long standing time management issue
Bug fixes for S3 (suspend to RAM) handling
Bug fixes for other low level system state handling
Bug fixes and improvements to the libxl tool stack
Bug fixes to nested virtualization

The Xen Project 4.2 release incorporates many new features and improvements to existing features. There are improvements across the board including to Security, Scalability, Performance and Documentation.

XL is now the default toolstack: Significant effort has gone in to the XL tool toolstack in this release and it is now feature complete and robust enough that we have made it the default. This toolstack can now replace xend in the majority of deployments, see XL vs Xend Feature Comparison. As well as improving XL the underlying libxl library has been significantly improved and supports the majority of the most common toolstack features. In addition the API has been declared stable which should make it even easier for external toolstack such as libvirt and XCP’s xapi to make full use of this functionality in the future.
Large Systems: Following on from the improvements made in 4.1 Xen now supports even larger systems, with up to 4095 host CPUs and up to 512 guest CPUs. In addition toolstack feature like the ability to automatically create a CPUPOOL per NUMA node and more intelligent placement of guest VCPUs on NUMA nodes have further improved the Xen experience on large systems. Other new features, such as multiple PCI segment support have also made a positive impact on such systems.
Improved security: The XSM/Flask subsystem has seen several enhancements, including improved support for disaggregated systems and a rewritten example policy which is clearer and simpler to modify to suit local requirements.
Documentation: The Xen documentation has been much improved, both the in-tree documentation and thewiki. This is in no small part down to the success of the Xen Document Days so thanks to all who have taken part.

You can find more information in the Xen 4.2 release notes, the Xen 4.2 feature list and the Also see the Xen 4.x Feature Matrix.
Xen Project Hypervisor 4.2 Acknowledgements
Contributions were made to this release by 124 individuals from 43 organizations, not counting contributions to external projects such as the BSDs, Linux or qemu. Many thanks to everyone who contributed to this release, either through code, testing, documentation or in any other way. For a complete breakdown of community contributions, see Xen 4.2 Acknowledgements.