The wm in twm and fvwm stands for window manager. A desktop environment contains a window manager, though its much more than merely that. Just like an apple is a fruit, but not every fruit is an apple. Comparing a DE with a WM is apples to oranges. You can probably look up the exact differences in Wikipedia.
One can try out Xfce on a Raspberry Pi as its the default DE on Raspbian (full default install). It is blazing fast. Since most people are accustomed to Windows 7 and Windows 10 (or Server equivalents) it works reasonably easy for them. You can't say the same for the two (or any) WMs you mention.
My default "WM" is a TUI called Tmux. I use it on every practically every computer I use.
For example, a desktop environment also offers public APIs for the applications to plug into, which XFCE doesn't seem to offer other than D-BUS.