The remote Debian host is missing a security-related update.
Multiple vulnerabilities have been discovered in the Xen hypervisor, which could result in denial of service, information leaks, privilege escalation or the execution of arbitrary code.
Upgrade the xen packages. For the oldstable distribution (jessie) a separate update will be released. For the stable distribution (stretch), these problems have been fixed in version 4.8.2+xsa245-0+deb9u1.