VirtIO-RNG has not been officially accepted into Xen (it seems to be easier to incorporate into a PV DOMU).
If you have RDRAND hardware, you could (I suppose) pipe it's output from DOM0 into an encrypted root terminal session on DOMU, and feed that into the pool.
I personally don't want to rely on RDRAND. Unlike a tampered ADD instruction, a tampered RDRAND instruction would appear to be undetectable, even in principle. I wish Intel had given me the raw bits from their DRBG, instead of providing only the whitened stream. Or hell, give me both - but not just an opaque stream.
If you have RDRAND hardware, you could (I suppose) pipe it's output from DOM0 into an encrypted root terminal session on DOMU, and feed that into the pool.
I personally don't want to rely on RDRAND. Unlike a tampered ADD instruction, a tampered RDRAND instruction would appear to be undetectable, even in principle. I wish Intel had given me the raw bits from their DRBG, instead of providing only the whitened stream. Or hell, give me both - but not just an opaque stream.