David Edmundson has long been working on LightDM-KDE: a version of the LightDM catered towards KDE. With Kubuntu 12.10 planning to use LightDM-KDE (the Unity/GNOME version of Ubuntu already uses LightDM), Edmundson is looking to make LightDM-KDE more official. Currently LightDM-KDE is living within KDE's Playground.
David writes on kde-core-devel, "It was always my intention to merge into KDE Workspaces, so the purpose of this thread is to make it clear that I want to do so and to work out together what steps still need to be done in order for that to happen. Obviously replacing any KDE application is a bit of a contentious issue, especially given that KDM still works adequately, though obviously I do feel mine is better. What I would like to do is discuss moving LightDM into KDE workspaces _alongside_ KDM and making compiling KDM optional, but keeping it enabled by default."
Among the benefits of LightDM-KDE over KVM is support for guest log-ins, use of KDE Plasma components, ability to make some customizations without XML editing, an easy-to-use KCM, suspending on lid close, is under active development, extensive unit tests upstream, and Wayland support is imminent.
Features though where KDM still comes in front is support for switching X sessions, being able to handling connections to remote XDMCP sessions, hot new stuff / theme installer, and support for rebooting into different GRUB entries.
Coming up for LightDM-KDE in the future is a big code refactor, better readying Wayland support by making X11-specific code in the greeter being a compile-time option, everything important KDM has, and better support for alternate log-in mechanisms.