Will all plain TeX code also work with XeTeX?
The question is not really well formed as plain tex is a macro format and xetex is an extended TeX engine, but I assume you mean to compare plain Tex with the tex (or pdftex) engine, with plain tex using xetex.
Most things will work, but some things will not, such as
\ifx\Umathchar\undefined hello \else \ERROR \fi \bye
hello with pdftex and just makes an error with xetex,
or this (latin-1, ISO 8859-1) encoded document
\font\x=ecrm1000\x café \bye
café with pdftex but with xetex typesets
caf and warns in the log:
Invalid UTF-8 byte or sequence at line 4 replaced by U+FFFD. Missing character: There is no � in font ecrm1000!