After installing the plymouth in my machine (Arch Linux 64), I'm facing this trouble.
tty1 is echoing every command that I type, even in the prompt, showing my password!
I don't have any idea in how to depurate this.
Can you give me a help?
It sounds like you're quitting plymouth after starting your getty, make sure you quit plymouth before starting your getty.
plymouth will restore tty settings to the way it found them when it quits. That could overwrite any settings your getty tries to change.
going to close this out, but please reopen if you continue to have problems or want to discuss this more.