What's the precise code you're running to set and get the value?

(In reply to Richard W.M. Jones from comment #1)
> What's the precise code you're running to set and get the value?

I set string "hello" with key='mykey' and type=1 in guestfish, and can find 'mykey' in windows registry, but the its value is gibberish not 'hello'.

Here's a "reproducer" (note: I don't think this is a bug).
You will need the 'minimal' file from hivex source here:
https://github.com/libguestfs/hivex/blob/master/images/minimal

#!/bin/bash -

guestfish -x -N fs -m /dev/sda1 <<EOF

upload /mnt/scratch/minimal /minimal
hivex-open /minimal write:true

hivex-root
# 0x1020

hivex-node-set-value 0x1020 mykey 1 hello

hivex-node-get-value 0x1020 mykey
# 0x2028

hivex-value-value 0x2028
hivex-value-utf8 0x2028

EOF

The problem is the following line:

  hivex-node-set-value 0x1020 mykey 1 hello

This sets the contents of the hive value to the bytes
"hello" (I believe without any trailing \0).  However
Windows would expect a UTF16-LE string here (hivex does
no conversion).  guestfish doesn't let you write a
string containing \0 escapes, but you could do it in
another language, eg in Perl:

#!/usr/bin/perl -w

use Data::Dumper;
use Sys::Guestfs;

my $disk = "/tmp/test.img";
my $minimal = "/mnt/scratch/minimal";
#my $value = "hello\0";
my $value = "h\0e\0l\0l\0o\0\0\0";

unlink $disk;
open DISK, ">$disk" or die "$!";
truncate DISK, 100*1024*1024 or die "$!";
close DISK or die "$!";

my $g = Sys::Guestfs->new ();
$g->set_trace (1);
$g->add_drive ($disk, format => "raw");
$g->launch ();
$g->part_disk ("/dev/sda", "mbr");
$g->mkfs ("ext2", "/dev/sda1");
$g->mount ("/dev/sda1", "/");
$g->upload ($minimal, "/minimal");
$g->hivex_open ("/minimal", write => 1);
my $root = $g->hivex_root ();
$g->hivex_node_set_value ($root, "mykey", 1, $value);
my $valh = $g->hivex_node_get_value ($root, "mykey");
printf ("raw value = %s\n", Dumper ($g->hivex_value_value ($valh)));
printf ("utf8 value = %s\n", Dumper ($g->hivex_value_utf8 ($valh)));

To the best of my knowledge, this is NOTABUG based
on the rationale in comment 3 & comment 4, therefore
I am closing this.