from typing import Dict


def get_short_namespace(full_ns: str, xml_namespaces: Dict[str, str]) -> str:
    """
    Inverse search of a short namespace by its full namespace value.

    :param full_ns: The full namespace of which the abbreviated namespace is to be found.
    :param xml_namespaces: A dictionary containing the mapping between short namespace (keys) and
                           long namespace (values).
    :return: If the full namespace is found in the dictionary, returns the short namespace.
    :raise KeyError: If the full namespace is not found in the dictionary.
    """
    for key, value in xml_namespaces.items():
        if full_ns == value:
            return key

    raise KeyError('The namespace is not found in "xml_namespaces".', full_ns)