I read the documentation. I was surprised to see an inbuilt terminal emulator (VT220/xterm) in neovim.
I wonder if it’s the best choice. What about other fast terminals like kitty, alacritty?
I feel there might be a misunderstanding here: neovim embeds a terminal emulator to allow you to operate on terminals as buffers, but you cannot use it to run neovim (well, maybe you could run a second instance within it, but the main instance would still need a different terminal).
I personally find wezterm to be the best compromise between speed and features, but both kitty and alacritty are also good choices. The only downside of alacritry is that it doesn’t support ligatures.
what if i want to open term buff inside neovim with kitty terminal not the embedded xterm one?
That doesn’t quite make sense. You could always run kitty from vim as an external program, but that would spawn another window which you couldn’t control from (neo)vim.
Using neovim’s terminal doesn’t embed a new graphical window, it simply provides the infrastructure to run a shell within a vim buffer. It is more akin to splitting your terminal window with tmux than to opening a new terminal window. Also, to be clear, it doesn’t actually embed xterm: it uses libvterm to provide an xterm-like implementation of a virtual terminal (presumably in a separate process, to keep things asynchronous). You cannot do this with kitty, and if you could you would still have to embed it as a library.
Edit: this Wikipedia page might help you understand the difference between a virtual terminal and a graphical terminal window.