When compiling with Xen support pulseaudio requires /usr/include/xenctrl.h which comes from app-emulation/xen-tools not app-emulation/xen as is the current RDEPEND.
whilst app-emulation/xen-tools is pulled in by app-emulation/xen it is only a PDEPEND so it will not necessarily be pulled in in-time for the pulse build.
simple patches for current 2.x & 3.x pulseaudio ebuilds attached.
this will cause a circular dependency on a vanilla system, since:
xen-tools depends libsdl depends pulseaudio depends xen-tools
a simple fix in that case being initially to USE=-xen emerge -av pulseaudio :)
Created attachment 338066 [details, diff]
pulseaudio 2.1 xen fix
Created attachment 338068 [details, diff]
pulseaudio 3.0 xen fix
fixed in 4.99.4