According to this
thread, AZW3-capable Kindles support Unicode-encoded AZW3 Hindi books.
I'd strongly recommend not to send Hindi books by e-mail to your Kindle because this might trigger another undesired automated conversion.
Simply transfer the books via the USB cable to your Kindle's document folder.
Note that if you see random spaces within words, the book was most likely not encoded in Unicode and references a non-Unicode font.