$host is a variable of the Core module.
$host
This variable is equal to line Host in the header of request or name of the server processing the request if the Host header is not available.
This variable may have a different value from $http_host in such cases: 1) when the Host input header is absent or has an empty value, $host equals to the value of server_name directive; 2)when the value of Host contains port number, $host doesn't include that port number. $host's value is always lowercase since 0.8.17.
$http_host is also a variable of the same module but you won't find it with that name because it is defined generically as $http_HEADER (ref).
$http_HEADER
The value of the HTTP request header HEADER when converted to lowercase and with 'dashes' converted to 'underscores', e.g. $http_user_agent, $http_referer...;
Summarizing:
$http_host equals always the HTTP_HOST request header.$host equals $http_host, lowercase and without the port number (if present), except when HTTP_HOST is absent or is an empty value. In that case, $host equals the value of the server_name directive of the server which processed the request.