The Xen Project 4.2 release contains a number of important new features and updates including:
The 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.
The diagram below shows organisations which contributed more than 1% in lines of code to the Xen Project 4.2 release. Several items in the diagram discribe groups of people or organisations: Individual covers contributions by individuals whose affiliation is unknown, Misc covers contributions by commercial organisations which did not go above 1% individually and University covers contributions by Universities which did not go above 1% individually.

For a complete breakdown of community contributions, see Xen 4.2 Acknowledgements.