When trying to build xorg-server with an nvidia video card, we got:

* Messages for package x11-base/xorg-server-1.5.3-r6:
 
 * Forcing on xorg-x11 for new enough glxtokens.h...
 * Users of reduced blanking now need:
 *    Option "ReducedBlanking"
 * In the relevant Monitor section(s).
 * Make sure your reduced blanking modelines are safe!
 * You must rebuild all drivers if upgrading from xorg-server 1.4.1
 * or earlier, because the ABI changed. If you cannot start X because
 * of module version mismatch errors, this is your problem.
 * You can generate a list of all installed packages in the x11-drivers
 * category using this command:
 * emerge portage-utils; qlist -I -C x11-drivers/
 
 * Messages for package x11-drivers/nvidia-drivers-180.60:
 
 * Please disable PARAVIRT_GUEST in your kernel config, found at:
 *
 *   Processor type and features
 *     [*] Paravirtualized guest support
 *
 * or XEN support
 *
 * and recompile your kernel ..
 *
 * ERROR: x11-drivers/nvidia-drivers-180.60 failed.
 * Call stack:
 *                      ebuild.sh, line   49:  Called pkg_setup
 *   nvidia-drivers-180.60.ebuild, line  195:  Called paravirt_check
 *   nvidia-drivers-180.60.ebuild, line  175:  Called die
 * The specific snippet of code:
 *           die "PARAVIRT_GUEST support detected!"
 *  The die message:
 *   PARAVIRT_GUEST support detected!
 *
 * If you need support, post the topmost build error, and the call stack if relevant.
 * A complete build log is located at '/var/tmp/portage/x11-drivers/nvidia-drivers-180.60/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/x11-drivers/nvidia-drivers-180.60/temp/die.env'.
 *
 

To fix this problem, just downgrade the driver by adding the following mask in /etc/portage/package.mask:

>=x11-drivers/nvidia-drivers-174.00