I have released to the master mirror the following packages for Xen4CentOS:

b518e3b0fd4f735b34d7784815793cb787283d9029776bd9794cedeb274b9204
kernel-3.18.21-16.el6.x86_64.rpm

4b17373a0ef6806b26d02eb87fc299020beb60b6e31ed08a9c57d30a1410fbe7
kernel-devel-3.18.21-16.el6.x86_64.rpm

cacedf6771849f7525145ad4ed7d36a2cad5b49b546173bb7c132b236cb7ac49
kernel-doc-3.18.21-16.el6.noarch.rpm

22479ee9c2d3748edb08b410514d12c17ad966bcc2c067c48deedf732ae62aae kernel-firmware-3.18.21-16.el6.noarch.rpm

5d7cd57276f8755c8c312149888956f230ed356bc753117365b4ad7df7b1bf69
kernel-headers-3.18.21-16.el6.x86_64.rpm

99f65bb828a4d1f9ee25c77d0c71b3d88eba93fcf3fb0babf6c10379d426e5b3
perf-3.18.21-16.el6.x86_64.rpm

616a1fd6689f214f10dce7b70eb1984e60371edb8341140ddd0b4850fa9705de xen-4.4.3-3.el6.x86_64.rpm

2a9584266af525126e7b6a2fa2eeac3757aa63bc6636a8393e0b65e6f4694a42
xen-devel-4.4.3-3.el6.x86_64.rpm

4ac658cd679ba54496ea82e018ec89156620b2e4c93ef93c3afcfd0956b22f9e xen-doc-4.4.3-3.el6.x86_64.rpm

ebfd368aa03a16a1c943afbda8795b631a88a800edcb0650903620928264ab48
xen-hypervisor-4.4.3-3.el6.x86_64.rpm

c2fa54596923d000e32cd5e96982a06b95ac9a9d1c20655ea3371d44620a2f0e xen-libs-4.4.3-3.el6.x86_64.rpm

a2481c5a3e55d20028f90c1b44195cfb64f22a34f9956ee67ce3a7d1bd9a24ea xen-licenses-4.4.3-3.el6.x86_64.rpm

283a3052c17735103aef249b3c183b5d9781f76f110dff3d585425af276d0e50
xen-ocaml-4.4.3-3.el6.x86_64.rpm

f2058a63167d3f0781ab71267b7c10132b18c836d7d4d8bd78214d88d930f5e3
xen-ocaml-devel-4.4.3-3.el6.x86_64.rpm

43ed7df8e5fb1e836e84f3c517726a6f0512ac6afbbc2eec4af76a39a8cb9eae xen-runtime-4.4.3-3.el6.x86_64.rpm

These changelog entries are applicable for the new kernel compared to the previously released version:
https://cdn.kernel.org/pub/linux/kernel/v3.x/ChangeLog-3.18.21
https://cdn.kernel.org/pub/linux/kernel/v3.x/ChangeLog-3.18.20
https://cdn.kernel.org/pub/linux/kernel/v3.x/ChangeLog-3.18.19
https://cdn.kernel.org/pub/linux/kernel/v3.x/ChangeLog-3.18.18

The xen updates roll in:

* Thu Oct 29 2015 George Dunlap –
4.4.3-3.el6.CentOS
– Update fix for XSA-150

* Thu Oct 22 2015 George Dunlap –
4.4.3-2.el6.CentOS
– Import XSAs 149-153

Thanks, Johnny Hughes