I don't like unity in new ubuntu. So I download xubuntu for my laptop. Now I don't like xfce as well and I want gnome. How can I completely remove xfce from xubuntu 13.04 and install gnome 3.8?
If you want gnome similar to as it was is Ubuntu 10.04 then you can install the Gnome Classic Desktop as described in this question:
Since 13.04 however there is an official variant of Ubuntu with Gnome as the default desktop
If you want this I recommend you do a backup and a clean install. Its a fully supported variant so if you have any problems you can ask about them here.
Start with installing "gnome-session" then you can change your desktop environment from xfce4 to gnome at log in time.
If you really want to remove xfce4 after trying gnome check out the instructions here. They were written for 12.10 but it will point you in the write direction.
Many of the packages you're instructed to remove are useful programs that work in gnome as well as xfce4 (such as abiword, gmusicbrowser to name two).
I'd start by limiting myself to removing packages named "xf*" or "xubuntu*". After remove those packages many orphaned libraries can be autoremoved.
Don't just cut and paste the instructions there or you'll end up installing Unity again by installing "ubuntu-desktop".