I'm not an ubuntu user, although I try out Xubuntu when there is a new one out, and ubuntu core 13.10 is actually pretty fast. Xubuntu and Lubuntu are very customizable, like most DE's in linux. But you're not limited to a specific DE or WM for a particular distro, you can install any DE or WM on any distro. Ubuntu (the version from Canonical with Unity as default DE), is pretty unstable as always when it first comes out, it's mostly pretty stable by the time it's replaced by the next version. The reason why Unity versions are extra unstable is because of XMir, the transitional Mir on X11 display server. Mir is Canonical-only, the community versions based on Ubuntu Core don't follow Canonical into Mir, they are following the rest of the linux distro world and going for Wayland with the compositor that is specific to the DE they default with, which is KWin for Kubuntu, Mutter or Weston for Ubuntu with Gnome, and Xubuntu and Lubuntu stay with X11 for the moment. Many people use XFCE now because it's well matured, is very customizable, gtk3 compatible and blazing fast, and there are no display server experiments yet.
For the future, if you're a graphics design person, check out the Hawaii DE. It's available for Fedora that I know off, and it fully integrates Wayland+Weston as far as that's possible for the moment. I think it will be a very important DE in the future because it looks great, is very lightweight, and is built from the start with Wayland+Weston in mind, instead of X11. But it's still very limited right now because Wayland+Weston is very limited.
LXC is a linux container. It's not really a high security solution, although on an SELinux distro it's more than secure enough for applications that are privacy-invasive only, but not malware-infected, and linux-native, like Skype, the Steam client, or the Chromium browser. KVM is full virtualization, also with hardware passthrough if your system supports it. KVM+QEMU can virtualize literally everything, even ARM, MIPS, other x86 systems, etc... e.g. you can virtualize an eight core CPU on a quad-core machine, you can run Android for ARM on your PC in a container, etc... KVM is very fast, and solves a lot of Windows bottlenecks, so most Windows games in a Windows KVM container with hardware virtualization and VGA or PCI passthrough, perform better in the container than on a bare metal windows install. If you have two graphics cards, you can skip the VGA passthrough (although it's possible with AMD GPU cards, but it requires some tinkering), and do a PCI-bind instead, which is a lot easier and also works with non-AMD graphics cards, whereby you reserve one card for the Windows container, and one card for the linux host system, and you switch between HDMI and DVI-D on your monitor to switch between linux and windows. That is the system many high performance gamers use. It is easy, if windows isn't running, you have two AMD cards in linux for extra OpenCL performance if you're using AMD cards, or some have an AMD card to use in linux for extra OpenCL performance, and an nVidia card to play nVidia driver optimized games in the Windows container, even if that nVidia card is pretty useless in linux because of lack of working drivers on modern kernels, lack of decent open source driver, and very bad OpenCL performance (except for some nVidia-specific benchmarks, those do exist, there are benchmarks for everything, but real-life OpenCL performance is pretty dismal with nVidia cards in linux, if the driver even works), but due to the software performance capping on nVidia-sponsored game titles for non-nVidia cards, gamers that play nVidia-sponsored games might want to use an nVidia card for those to get decent performance.