"I wonder about the future where the main OS of a notebook/PC is a VM OS (something like Xen), where we could choose to boot up multiple OSes and communication/sharing between these OS are still possible, and the VM OS doesn't consume too much memory and CPU."
Windows 8 ships with the Hyper-V hypervisor, and provided one's computer has a recent CPU, begins to fulfill the author's vision.
If you had asked me 5 years ago, I would have also been hyped about using a hypervisor to run Windows alongside GNU/Linux, but nowadays I am far more interested in GNU/Linux getting triple A video game titles so that I can simply abandon Windows. When you dual-boot eventually you get sick of powering on/off and you spend most of your time in one of the OSs; I never got to play my games anymore. My solution was to do development on a GNU/Linux or Mac laptop and leave Windows+cygwin on my gaming rig. I frequently use synergy[1] to share my gaming rig's mouse, keyboard, and copy/paste buffer with my laptop over WiFi.
My solution was to run Windows and a very lightweight VM (~512Mb RAM) with a minimal development setup (Arch + emacs + Xmonad) which can be deployed anywhere. RAM can be expanded anytime to allow running heavy servers (such as MongoDB) but the light VM is usually just fine.
I also tried VPS servers on demand (Digital Ocean) + PuTTy on Windows to do remote development, but it isn't nearly as comfortable. Dealing with network waits (e.g. sending large file transfers) was awful, so I don't do that anymore.
I did this because I mostly use Linux for development and Windows for everyday tasks (maintaining a Linux system is tiring even with Ubuntu).
I thought about setting a dev server like you, but I don't want yet another machine in my room wasting space and generating heat. Light ARM systems (e.g. RasPI) might be a good alternative, but I don't feel like dealing with proprietary platforms right now.
I believed Hyper V is more suited if the machine is a server hosting the VM, where client remote into this server from another machine (correct me if I am wrong; perhaps Xen works the same way as well).
It would be nice to get some suggestions on Minimalist/Lightweight Host OS to run VirtualBox. Lubuntu?
Windows 8 ships with the Hyper-V hypervisor, and provided one's computer has a recent CPU, begins to fulfill the author's vision.