Version 1.0.0 doesn't represent some fundamental change in the library used for managing Linux virtual machines but rather is a symbolic milestone for celebrating its seventh birthday. This birthday celebration decision was shared on the blog of Daniel Berrangé. The team has also made a Gource video to show off libvirt's development over the past seven years (embedded at the end of this article).
Changes for the libvirt 1.0.0 release are outlined at libvirt.org. Among the small features to this new release include systemd journal support, a QEMU capabilities cache manager, USB migration support, QMP QEMU Interface Support for Xen 4.2, and lots of localization enhancements. The 1.0.0 release also carries many bug-fixes, documentation updates, etc.