The Citrix office in Cambridge develops the XenServer hypervisor which is open source and builds on Xen. The toolstack part of the product, which orchestrates the VMs, is developed in OCaml (see GitHub and the team is looking for an engineer with strong affinity to functional programming in the Unix environment. Feel free to send me an email at christian.lindig@citrix.com with any questions. Below is the official job description.
https://jobs.citrix.com/job/CITRA005811517/Software-Engineer-Functional-Programming