Paolo Bonzini [off-list ref] writes:

CONFIG_HYPERV=m is possible.

We've stubmled upon this multiple times already. Initially, the whole
Hyper-V support code was a module (what's now in drivers/hv/) but then
some core functionallity was moved out to arch/x86/ but we didn't add a
new config back then. Still suffering :-)

Ideally, we would want to have 

CONFIG_HYPERV_GUEST=y/n for what's in arch/x86/ (just like CONFIG_KVM_GUEST)
CONFIG_HYPERV_VMBUS=y/n/m for what's in drivers/hv

-- 
Vitaly