How to get UTF8 from a hex variable?

With xxd (usually shipped with vim)

$ echo 5374c3a97068616e650a | xxd -p -r
Stéphane

If your locale's charset (see output of locale charmap) is not UTF-8 (but can represent all the characters in the encoded string), add a | iconv -f UTF-8.

If it cannot represent all the characters, you could try | iconv -f UTF-8 -t //TRANSLIT to get an approximation.

Does Perl count?

$ echo "68656c6c6f0a" | \
  perl -ne 'tr/a-fA-F0-9//cd; print pack("H*", $_)'
hello

If not, then maybe this might do:

$ echo "68656c6c6f0a" | sed -Ee 's/[0-9a-fA-F]{2}/\\\\x&/g' | xargs printf 
hello

We need a literal backslash for printf, but it's special for both xargs and sed so needs to be doubled twice. (\\\\x -> \\x -> \x)